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Humanities Libertexts

Immanuel Kant (1724–1804)

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  • Born in Königsberg, Prussia, Immanuel Kant is the only writer included in this book to have been a philosophy professor. Kant published his massive Critique of Pure Reason (CPR) in 1781, which he revised in 1787. In between, he wrote the Prolegomena (1783). As you’ll see, Kant was stung by the critical reception of the Critique; the Prolegomena is intended to serve as a kind of introduction to the Critique. I’ve interspersed relevant parts of the Critique within the text of the Prolegomena below.

    Here, by section number, is an outline of the Prolegomena, using Kant’s own headings. I’ve also indicated where i’ve introduced material from the CPR.

    From the CPR: Transcendental Deduction

    (Textual note: the standard translations of Kant’s work are in the Cambridge Edition of the Works of Immanuel Kant in Translation. Norman Kemp Smith’s translation of the Critique of Pure Reason is still very useful, however.)

    Prolegomena to Every Future System of Metaphysics Which Can Claim to Rank as Science, Introduction

    … My purpose is to convince all those who care to trouble themselves with metaphysics that it is indispensably necessary for the present to suspend their work, to look upon all that is gone before as non-existent, and, above all things, first to propose the question “Whether such a thing as metaphysics be even possible at all?”

    If it be a science, how comes it that it cannot like other sciences win for itself a universal and lasting recognition? If it be not one, how is it that under the semblance of a science it is ceaselessly boasting and holding out to the human understanding hopes that are never extinguished and never fulfilled? Something must be definitely decided respecting the nature of this assumed science, whether it be to demonstrate our knowledge or our ignorance; for it is impossible that it should remain longer on the same footing as heretofore. It seems nearly ridiculous, while every other science ceaselessly progresses, that this which is supposed to be wisdom itself, whose oracle every one interrogates, is continually turning round on the same spot, without moving a step in advance. Its votaries have also much decreased, and we do not see those who feel themselves strong enough to shine in other sciences, willing to risk their fame in this, where every one, ignorant though he be in all else, ventures upon a decided opinion, because in this sphere there is no certain weight and measure at hand by which to distinguish profundity from worthless jargon.

    It is, however, not unheard of, after lengthened treatment of a science, when wonders are thought as to the progress made in it, that some one lets fall the question: Whether and how such a science is possible at all? For the human Reason is so fond of building, that it has many times reared up a lofty tower and afterwards pulled it down again, to see how its foundation was laid. It is never too late to become reasonable and wise; but it is always more difficult when the knowledge comes late to bring it into working order.

    To ask, whether a science is possible presupposes a doubt as to its reality. But such a doubt must offend all those whose whole fortune, perhaps, consists in this supposed treasure; any one who starts such a doubt may always make up his mind then for resistance on all sides. Some, in the proud consciousness of their old and therefore, as they think, legitimate possession, with their metaphysical compendiums in their hands, will look down upon it with contempt. Others, who never see anything anywhere that does not coincide with what they have elsewhere previously seen, will not understand it, and everything will remain for some time as though nothing at all had happened to prepare or to admit the hope of a near change.

    At the same time, I may confidently predict that the open-minded reader of these Prolegomena will not merely doubt his previous science, but in the end will be quite convinced, that there cannot exist such a science without the demands here made being satisfied, upon which its possibility rests, and that inasmuch as this has never happened, that there is as yet no such thing as metaphysics at all. But as notwithstanding the search after it can never lose its interest, because the interests of the universal human Reason are so intimately bound up with it, he will confess that a complete reform, or rather a new birth according to a plan hitherto quite unknown, is inevitable, however much it may be striven against for a time.

    Since the attempts of Locke and Leibniz, or rather since the first rise of metaphysics as far as its history will reach, no event has occurred that in view of the fortunes of the science could be more decisive than the attack made upon it by David Hume. He threw no light upon this order of knowledge, but he struck a spark by which a light might have been kindled, had it touched a receptive substance, to have preserved and enlarged its glimmer.

    Hume took for his starting-point, mainly, a single but important conception of metaphysics, namely, that of the connection of Cause and Effect (together with the derivative conceptions of Force and Action, &c.) and required of the Reason which professes to have given it birth a rigid justification of its right, to think, that something is so constructed that on its being posited something else is therewith necessarily also posited; for so much is contained in the conception of Cause. He proved irrefutably that it is quite impossible for Reason a priori, out of mere conceptions, to cogitate this connection, since it involves necessity; but the problem nevertheless was not to be overlooked, how that, because something exists, something else must necessarily also exist, and thus how the conception of such a connection can be regarded as a priori. Hence he concluded that Reason completely deceived itself with this conception, that it falsely claimed it as its own child, while it was nothing more than a bastard of the imagination, which, impregnated by experience, had brought certain presentations under the law of association, and had substituted a subjective necessity arising thence, i.e., from habit, for an objective one founded on insight. From this he concluded that Reason possessed no faculty of cogitating such connections even in general, because its conceptions would then be mere inventions, and all its pretended a priori cognitions nothing but common experiences mislabelled; which is as much as to say, no such thing as metaphysics exists at all, and there is no possibility of its ever existing.

    However hasty and incorrect his conclusion may have been, it was at least based on investigation, and it would have been well worth while if the good heads of his time had united to solve the problem in the sense in which he had stated it, if as far as possible with happier results; the consequence of which must have been a speedy and complete reform of the science.

    But the always unfavourable fate of metaphysics, willed that he should be understood by no one. It is positively painful to see how completely his opponents, Reid, Oswald, Beattie, and, lastly, Priestley, missed the point of his problem in taking that for granted which was precisely what he doubted, and on the other hand in proving with warmth, and in most cases great immodesty, what it had never entered his head to question, and as a result in so completely mistaking his reforming hint that everything remained in the same state as though nothing had happened. It was not the question whether the conception of Cause was correct and useful, and in view of the whole knowledge of Nature, indispensable, for upon this Hume had never cast a doubt, but whether it could be cogitated a priori by Reason in such a manner as to constitute an inward truth independent of all experience, and therefore of a more extended use than that of being solely applied to the objects of experience; it was upon this that Hume desired enlightenment. The question was as to the origin of the idea, not as to its practical necessity in use; were the former ascertained, the conditions of its use and the extent in which it is valid would have been sufficiently obvious.

    The opponents of this celebrated man, to have done the problem full justice, must have penetrated deeply into the nature of Reason, in so far as it is occupied solely with pure thought, a thing which was inconvenient for them. They invented therefore a more convenient means, by which, without any insight, they might defy him, namely, the appeal to the common sense of mankind. It is indeed a great natural gift to possess, straightforward (or, as it has been recently called, plain) common sense. But it must be proved by deeds, by the thoughtfulness and rationality of what one thinks and says, and not by appealing to it as an oracle, when one has nothing wise to adduce in one’s justification. When insight and science are at a low ebb, then and not before to appeal to common sense is one of the subtle inventions of modern times, by which the emptiest talker may coolly confront the profoundest thinker and hold out against him. But so long as there is a small remnant of insight left, one will be cautious of clutching at this straw. And seen in its true light, the argument is nothing better than an appeal to the verdict of the multitude; a clamour before which the philosopher blushes, and the popular witling scornfully triumphs. But I should think that Hume can make as good claim to the possession of common sense as Beattie, and in addition, to something the latter certainly did not possess, namely, a critical Reason, to hold common sense within bounds in order not to let it overreach itself in speculations; or if we are merely concerned with the latter, not to require it to decide, seeing that it is incompetent to deal with matters outside its own axioms; for only in this way will it remain a healthy common sense. Chisel and hammer are quite sufficient to shape a piece of deal, but for copper-engraving an etching-needle is necessary. In the same way, common, no less than speculative understanding, is useful in its kind; the former when we have to do with judgments having an immediate bearing on experience, but the latter, where we have to judge, universally, out of mere conceptions, as for instance in metaphysics, where the self-styling (though often per antiphrasin) healthy understanding is capable of no judgment at all.

    I readily confess, the reminder of David Hume was what many years ago first broke my dogmatic slumber, and gave my researches in the field of speculative philosophy quite a different direction. I was far enough removed from giving him an ear so far as his consequences were concerned, the latter resulting merely from his not having placed his problem fully before him, but only attacking a part of it, which, without taking the whole into consideration, could not possibly afford a solution. When one starts from a well-founded, though undeveloped, idea that a predecessor has left, one may well hope, by increased reflection, to bring it further than was possible for the acute man one has to thank for the original sparks of its light.

    First of all, I tried whether Hume’s observation could not be made general, and soon found that the conception of the connection of cause and effect was not by a long way the only one by which the understanding cogitates a priori the connections of things, but that metaphysics consists entirely of such. I endeavoured to ascertain their number, and as I succeeded in doing this to my satisfaction, namely, out of a single principle, I proceeded to the deduction of these conceptions, which I was now assured could not, as Hume had pretended, be derived from experience but must have originated in the pure understanding. This deduction, that seemed impossible to my acute predecessor, that had not even occurred to any one except him, although every one unconcernedly used the conception (without asking on what its objective validity rested); this, I say, was the most difficult problem that could ever be undertaken in the interests of metaphysics, and the worst of it was, that metaphysics, so far as it anywhere exists at present, could not afford me the least help, because the above deduction had in the first place to make metaphysics possible. Having now succeeded in the solution of Hume’s problem, not in one particular case only, but in respect of the whole capacity of pure Reason, I could at least more surely, though still only by slow steps, determine the whole range of pure Reason, in its limits as well as in its content, completely according to universal principles, which was what metaphysics required, in order to construct its system on an assured plan.

    I am afraid, however, lest the carrying out of the problem of Hume in its greatest possible development (namely, in the Critique of Pure Reason) should fare as the problem itself fared when it was first stated. It will be falsely judged, because it is misunderstood; it will be misunderstood, because people, though they may care to turn over the leaves of the book, will not care to think it out; and they will be unwilling to expend this trouble upon it because the work is dry, obscure, and opposed to all accustomed conceptions, besides being diffuse. But I must confess, it was quite unexpected for me to hear from a philosopher complaints as to its want of popularity, entertainingness, and agreeable arrangement, when the question was of a branch of knowledge highly prized and indispensable to humanity, and which cannot be treated otherwise than according to the most strict rules of scholastic precision; whereby popularity may indeed follow in time, but can never be expected at the commencement. As regards a certain obscurity, however, arising partly from the diffuseness of the plan, in consequence of which the main points of the investigation are not so readily grasped, the grievance must be admitted, and this it is the task of the present Prolegomena to remove.

    The above work, which presents the capacity of pure Reason in its whole range and boundaries, always remains the foundation to which the Prolegomena are only preparatory; for the Critique must, as science, stand complete and systematic even down to the smallest detail, before we can so much as think of the rise of metaphysics, or even allow ourselves the most distant hope in this direction.

    We have been long accustomed to see old and worn-out branches of knowledge receive a new support, by being taken out of their former coverings, and suited with a systematic garment according to our own approved style, but under new titles; and the great majority of readers will expect nothing different from our Critique. But these Prolegomena will convince him that it is quite a new science, of which no one previously had had the smallest conception, of which even the idea was unknown, and with reference to which all hitherto received knowledge was unavailable, with the exception of the hint afforded by Hume’s doubt. But Hume never dreamt of a possible formal science of this nature, and in order to land his ship in safety, ran it aground on the shore of scepticism, where it might lie and rot; instead of which, it is my purpose to furnish a pilot, who, according to certain principles of seamanship, derived from a knowledge of the globe, and supplied with a complete map and compass, may steer the ship with safety wherever it seems good to him. …


    1. As Kant reads him, what exactly was Hume calling into question?
    2. Is Hume’s problem merely about causation, or does it extend to other concepts? Why, or why not?

    Prolegomena, Introductory Remarks on the Speciality of All Metaphysical Knowledge

    Section One: Of the Source of Metaphysics

    In presenting a branch of knowledge as science, it is necessary to be able to define with precision its distinguishing characteristic, that which it possesses in common with no other branch, and which is therefore special to itself; when this is not the case the boundaries of all sciences run into one another, and no one of them can be thoroughly treated of, according to its own nature.

    Now this speciality may consist in the distinction of its object, of its sources of cognition, of its mode of cognition, or lastly, of several if not all these points taken together, on which the idea of a possible science and of its territory primarily rests.

    Firstly, as regards the sources of metaphysical knowledge, the very conception of the latter shows that these cannot be empirical. Its principles (under which not merely its axioms, but also its fundamental conceptions are included) must consequently never be derived from experience; since it is not physical but metaphysical knowledge, i.e., knowledge beyond experience, that is wanted. Thus neither external experience, the source of physical science proper, nor internal experience, the groundwork of empirical psychology, will suffice for its foundation. It consists, then, in knowledge a priori, that is, knowledge derived from pure understanding and pure reason. …

    Section Two: Of the Mode of Cognition that can Alone be Termed Metaphysical

    1. Of the distinction between synthetic and analytic judgments generally.

      Metaphysical knowledge must contain simply judgments a priori, so much is demanded by the speciality of its sources. But judgments, let them have what origin they may, or let them even as regards logical form be constituted as they may, possess a distinction according to their content, by virtue of which they are either simply explanatory and contribute nothing to the content of a cognition, or they are extensive, and enlarge the given cognition; the first may be termed analytic, and the second synthetic judgments.

      Analytic judgments say nothing in the predicate, but what was already cogitated in the conception of the subject, though perhaps not so clearly, or with the same degree of consciousness. When I say, all bodies are extended, I do not thereby enlarge my conception of a body in the least, but simply analyse it, inasmuch as extension, although not expressly stated, was already cogitated in that conception; the judgment is, in other words, analytic. On the other hand, the proposition, some bodies are heavy, contains something in the predicate which was not already cogitated in the general conception of a body; it enlarges, that is to say, my knowledge, in so far as it adds something to my conception; and must therefore be termed a synthetic judgment.

    2. The common principle of all analytic judgments is the principle of contradiction.

      All analytic judgments are based entirely on the principle of contradiction, and are by their nature cognitions a priori, whether the conceptions serving as their matter be empirical or not. For inasmuch as the predicate of an affirmative analytic judgment is previously cogitated in the conception of the subject, it cannot without contradiction be denied of it; in the same way, its contrary, in a negative analytic judgment, must necessarily be denied of the subject, likewise in accordance with the principle of contradiction. It is thus with the propositions—every body is extended; no body is unextended (simple). For this reason all analytic propositions are judgments a priori, although their conceptions may be empirical. Let us take as an instance the proposition, gold is a yellow metal. Now, to know this, I require no further experience beyond my conception of gold, which contains the propositions that this body is yellow and a metal; for this constitutes precisely my conception, and therefore I have only to dissect it, without needing to look around for anything elsewhere.

    3. Synthetic judgments demand a principle other than that of contradiction.

      There are synthetic judgments a posteriori whose origin is empirical; but there are also others of an a priori certainty, that spring from the Understanding and Reason. But both are alike in this, that they can never have their source solely in the axiom of analysis, viz., the principle of contradiction; they require an altogether different principle, notwithstanding that whatever principle they may be deduced from, they must always conform to the principle of contradiction, for nothing can be opposed to this principle, although not everything can be deduced from it. I will first of all bring synthetic judgments under certain classes.

      1. Judgments of experience are always synthetic. It would be absurd to found an analytic judgment on experience, as it is unnecessary to go beyond my own conception in order to construct the judgment, and therefore the confirmation of experience is unnecessary to it. That a body is extended is a proposition possessing a priori certainty, and no judgment of experience. For before I go to experience I have all the conditions of my judgment already present in the conception, out of which I simply draw the predicate in accordance with the principle of contradiction, and thereby at the same time the necessity of the judgment may be known, a point which experience could never teach me.
      2. Mathematical judgments are in their entirety synthetic. This truth seems hitherto to have altogether escaped the analysts of human Reason; indeed, to be directly opposed to all their suppositions, although it is indisputably certain and very important in its consequences. For, because it was found that the conclusions of mathematicians all proceed according to the principle of contradiction (which the nature of every apodictic certainty demands), it was concluded that the axioms were also known through the principle of contradiction, which was a great error; for though a synthetic proposition can be viewed in the light of the above principle, it can only be so by presupposing another synthetic proposition from which it is derived, but never by itself.

        It must be first of all remarked that essentially mathematical propositions are always a priori, and never empirical, because they involve necessity, which cannot be inferred from experience. Should any one be unwilling to admit this, I will limit my assertion to pure mathematics, the very conception of which itself brings with it the fact that it contains nothing empirical, but simply pure knowledge a priori.

        At first sight, one might be disposed to think the proposition 7+5=12 merely analytic, resulting from the conception of a sum of seven and five, according to the principle of contradiction. But more closely considered it will be found that the conception of the sum of 7 and 5 comprises nothing beyond the union of two numbers in a single one, and that therein nothing whatever is cogitated as to what this single number is, that comprehends both the others. The conception of twelve is by no means already cogitated, when I think merely of the union of seven and five, and I may dissect my conception of such a possible sum as long as I please, without discovering therein the number twelve. One must leave these conceptions, and call to one’s aid an intuition corresponding to one or other of them, as for instance one’s five fingers (or, like Segner in his Arithmetic, five points), and so gradually add the units of the five given in intuition to the conception of the seven. One’s conception is therefore really enlarged by the proposition 7+5=12; to the first a new one being added, that was in nowise cogitated in the former; in other words, arithmetical propositions are always synthetic, a truth which is more apparent when we take rather larger numbers, for we must then be clearly convinced, that turn and twist our conceptions as we may, without calling intuition to our aid, we shall never find the sum required, by the mere dissection of them.

        Just as little is any axiom of pure geometry analytic. That a straight line is the shortest between two points, is a synthetic proposition. For my conception of straight, has no reference to size, but only to quality. The conception of the “shortest” therefore is quite additional, and cannot be drawn from any analysis of the conception of a straight line. Intuition must therefore again be taken to our aid, by means of which alone the synthesis is possible.

        Certain other axioms, postulated by geometricians, are indeed really analytic and rest on the principle of contradiction, but they only serve, like identical propositions, as links in the chain of method, and not themselves as principles; as for instance a=a, the whole is equal to itself, or (a+b) > a, i.e., the whole is greater than its part. But even these, although they are contained in mere conceptions, are only admitted in mathematics because they can be presented in intuition. What produces the common belief that the predicate of such apodictic judgments lies already in our conception, and that the judgment is therefore analytic, is merely the ambiguity of expression. We ought, namely, to cogitate a certain predicate to a given conception, and this necessity adheres even to the conceptions themselves. But the question is not what we ought to, but what we actually do, although obscurely, cogitate in them; this shows us that the predicate of those conceptions is dependent indeed necessarily, though not immediately (but by means of an added intuition), upon its subject.

    Section Three: Observation on the Universal Division of Judgments into Analytic and Synthetic

    This division is in view of the Critique of human understanding indispensable, and deserves therefore to be classic in this department; though I am not aware of any other in which it has any important use. And here I also find the cause why dogmatic philosophers who looked for the sources of metaphysical judgments in metaphysics itself (rather than outside of it, in the laws of pure Reason in general), have always neglected this division, that seems so naturally to offer itself, and like the celebrated Wolff, or the acute Baumgarten, who followed in his steps, have sought the proof of the principle of sufficient reason, which is obviously synthetic, in that of contradiction. On the other hand, I can trace already in Locke’s Essay Concerning Human Understanding a notion of this division. For in the third chapter of the fourth book, (Chapter Three, Section Nine, et seq.,) after he has spoken of the connection of different presentations in judgments, and of their sources, one of which he places in identity or contradiction (analytic judgments), and the other in the existence of presentations in a subject (synthetic judgments), he confesses (Section Ten) that our knowledge (a priori) of the last is very limited, amounting almost to nothing. But there is so little that is definite and reduced to rule in what he says respecting this kind of knowledge, that one cannot wonder that nobody, strange to say, not excepting hume, was induced thereby to institute investigations into the class of propositions in question. For universal yet definite principles like these, are not easily learnt from other men, to whom they have been only dimly discernible. One must, first of all, have come upon them through one’s own reflection, and one will then find them elsewhere, in places where otherwise they would certainly not have been discovered; since not even the authors knew that such an idea lay at the foundation of their own remarks. …

    Section Four: The General Question of the Prolegomena: Is Metaphysics possible at all?

    Were metaphysics actually present as a science, one might say: Here is metaphysics, you only require to learn it, and it will convince you permanently and irresistibly of its truth. In that case the present question would be unnecessary, and there would only remain one which would more concern a testing of our acuteness, than a proof of the existence of the thing itself; namely, the question, How is it possible, and how is Reason to set about attaining it? Unfortunately, in this case, human Reason is not in such a happy position. There is no single book that can be shown, like for instance Euclid, of which it can be said: This is metaphysics, herein is to be found the chief end of the science, the knowledge of a Supreme Being and of a future world, demonstrated upon principles of pure Reason. It is possible, doubtless, to bring forward many propositions that are apodictically certain, and that have never been contested; but these are in their entirety analytic, and concern more the materials and the elements of construction, than the extension of knowledge, which is our special object in the present case. But even when synthetic propositions are produced (such as the principle of sufficient Reason), which though they have never been proved from mere Reason, that is, a priori, as they ought to have been, are willingly admitted; even then, whenever it is attempted to make use of them for the main purpose, one is landed in such unstable and doubtful assertions, that it has always happened that one system of metaphysics has contradicted another, either in respect of the assertions themselves or their proofs, and has thus destroyed all claim to a lasting recognition. The very attempts made to establish the science have without doubt been the primary cause of the scepticism that so early arose, a mode of thought in which Reason treats itself with such violence, that it would never have arisen but from the latter’s utter despair of satisfying its chief aspirations. For long before man began methodically to question Nature, he interrogated his own isolated Reason, already practised, in a measure by common experience; because Reason is always present, while the laws of Nature generally require to be laboriously sought out. And so metaphysics floated to the surface like foam, and like foam, too, no sooner was it gathered up than it dissolved, while another mass of it appeared upon the scene which some were always found eager to grasp; while others, instead of seeking to penetrate the cause of the phenomenon in question, thought themselves wise in laughing at the futile exertions of the former.

    The essential feature distinguishing pure mathematical knowledge from all other knowledge a priori, is that it does not proceed from conceptions themselves, but always through the construction of conceptions. (Critique, p. 435.) Since, therefore, in its propositions it must pass out of the conception to that containing the corresponding intuition, these can and ought never to arise from the dissection of conceptions, that is, analytically; in other words, they are, in their entirety, synthetic.

    I cannot refrain from remarking on the disadvantage resulting to philosophy from a neglect of this simple and apparently insignificant observation. Hume, indeed, feeling it a task worthy of a philosopher, cast his eye over the whole field of pure knowledge a priori in which the human understanding claims such extensive possession. He, however, inconsiderately severed from it an entire, and indeed the most important, province, namely, that of pure mathematics, under the impression that its nature, and, so to speak, its constitution, rested on totally different principles, that is, solely on the principle of contradiction; and although he did not make such a formal and universal division of propositions as is here done by me, or under the same name, yet it was as good as saying, pure mathematics contains simply analytic judgments, but metaphysics, synthetic judgments a priori. Now in this he made a great mistake, and this mistake had decidedly injurious consequences on his whole conception. For if he had not made it, he would have extended his question respecting the origin of our synthetic judgments far beyond his metaphysical conception of causality, and comprehended therein the possibility of mathematics a priori; for he must have regarded this as equally synthetic. But in the latter case he could, under no circumstances, have based his metaphysical propositions on mere experience, as he would then have been obliged to have subordinated the axioms of pure mathematics themselves to experience, a proceeding for which he was much too penetrating.

    The good company into which metaphysics would then have been brought must have ensured it against contemptuous treatment; for the strokes aimed at the latter must have also hit the former, and this neither was nor could have been his intention. The result must have been to lead the acute man to considerations similar to those with which we are now occupied, but which must have gained infinitely by his inimitable style.

    Essentially metaphysical judgments are, in their entirety, synthetic. We must distinguish between judgments belonging to metaphysics from metaphysical judgments proper. Among the former are comprised many that are analytic, but they only furnish the means for metaphysical judgments, these forming the entire purpose of the science, and being all synthetic. For when conceptions belong to metaphysics, as, for instance, that of substance, the judgments arising from their dissection belong also to metaphysics; e.g., substance is that which only exists as subject, &c., and many more similar analytic judgments, by means of which an endeavour is made to approach the definition of the conception. Since, however, the analysis of a pure conception of the understanding (such as those metaphysics contains) cannot proceed differently from the analysis of any other conception (even an empirical one) not belonging to metaphysics (e.g., air is an elastic fluid, the elasticity of which is not destroyed by any known degree of cold), it follows that the conception but not the analytic judgment, is properly metaphysical. The science in question has something special and peculiar in the production of its cognitions a priori, which must be distinguished from what it has in common with all other cognitions of the understanding; so, for instance, the proposition, “all that is substance in things is permanent,” is a synthetic and properly metaphysical judgment.

    When the conceptions a priori constituting the materials of metaphysics have been previously collected according to fixed principles, the dissection of these conceptions is of great value. They can be then presented as a special department (as it were a philosophia definitiva), containing solely analytic propositions relating to metaphysics, though quite distinct from the synthetic, which constitute metaphysics itself. For, indeed, these analyses have nowhere any important use, except in metaphysics, that is, in reference to the synthetic propositions, to be generated from these dissected conceptions.

    The conclusion drawn in this section is then, that metaphysics is properly concerned with synthetic propositions a priori, and that these alone constitute its purpose, but that, in addition to this, it requires frequent dissections of its conceptions, or analytic judgments, the procedure in this respect being only the same as in other departments of knowledge, where conceptions are sought to be made plain by analysis. But the generation of knowledge a priori, as much in intuition as in conceptions, in fine, synthetic propositions a priori in philosophical cognitions, make up the essential content of metaphysics.

    Wearied, then, of the dogmatism that teaches us nothing, as well as of the scepticism that promises us nothing, not even the rest of a permissible ignorance, led on by the importance of the knowledge we need, rendered mistrustful by a long experience, of all we believe ourselves to possess, or that offers itself in the name of pure Reason, there only remains one critical question, the answer to which must regulate our future procedure—Is metaphysics possible at all? But this question must not be answered by sceptical objections to particular assertions of any actual system of metaphysics (for we do not admit any at present), but from the, as yet, only problematical conception of such a science.

    In the Critique of Pure Reason, I went synthetically to work in respect of this question, in instituting researches into pure Reason itself, and in this source endeavoured to determine the elements, as well as the laws of its pure use, according to principles. The task is difficult, and demands a resolute reader, gradually to think out a system, having no datum other than Reason itself, and which, therefore, without supporting itself on any fact, seeks to unfold knowledge from its original germs. Prolegomena should, on the contrary, be preparatory exercises, designed more to show what has to be done, to realise a science as far as is possible, than to expound one. They must, therefore, rely on something known as trustworthy, from which we may with confidence proceed, and ascend to its sources, as yet unknown to us, and the discovery of which will not only explain what we already knew, but at the same time exhibit to us a range of many cognitions, all arising from these same sources. The methodical procedure of Prolegomena, especially of those destined to prepare a future system of metaphysics, will therefore be analytic.

    Now it fortunately happens that, although we cannot accept metaphysics as a real science, we may assert with confidence that certain pure synthetic cognitions are really given a priori, namely, pure mathematics and pure natural science, for both contain propositions, partly apodictically certain through mere Reason, and partly recognised by universal consent as coming from experience, and yet as completely independent of it.

    We have, then, at least some uncontested, synthetic knowledge a priori, and do not require to ask whether this is possible, since it is actual, but only—How it is possible, in order to be able to deduce from the principle, rendering possible what is already given, the possibility of all the rest.

    1. Kant’s ‘regressive’ method in the Prolegomena can be called a defense of metaphysics by means of a ‘companions in virtue’ strategy. Kant first identifies a key feature of metaphysical claims—they are synthetic and a priori—and then looks to see whether some more established sciences, whose existence and legitimacy we take for granted, share this feature. What other sciences fall into this category?

    Section Five: General Question: How is Knowledge from Pure Reason possible?

    We have already seen the important distinction between analytic and synthetic judgments. The possibility of analytic propositions can be very easily conceived, for they are based simply on the principle of contradiction. The possibility of synthetic propositions a posteriori, i.e., of such as are derived from experience, requires no particular explanation, for experience is nothing more than a continual adding together (synthesis) of perceptions. There remains, then, only synthetic propositions a priori, the possibility of which has yet to be sought for, or examined, because it must rest on other principles than that of contradiction.

    But we do not require to search out the possibility of such propositions, that is, to ask whether they are possible, for there are enough of them, actually given, and with unquestionable certainty; and as the method we are here following is analytic, we shall assume at the outset that such synthetic but pure knowledge from Reason, is real; but thereupon we must investigate the ground of this possibility and proceed to ask—How is this knowledge possible? in order that, from the principles of its possibility, we may be in a position to determine the conditions, the scope, and limits of its use. The proper problem, on which everything turns, when expressed with scholastic precision, will accordingly stand thus—How are synthetic propositions a priori possible?

    In the above, for the sake of popularity, I have expressed the question somewhat differently, namely, as an inquiry after knowledge from pure Reason, which I could do on this occasion without detriment to the desired insight. For as we are here simply concerned with metaphysics and its sources, I hope, after the above remarks, readers will constantly bear in mind that, when we here speak of knowledge from pure Reason, we invariably refer to synthetic and never to analytic knowledge. Upon the solution of this problem, the standing or falling of metaphysics, in other words, its very existence, entirely depends. Let any one lay down assertions, however plausible, with regard to it, pile up conclusions upon conclusions to the point of overwhelming, if he has not been able first to answer satisfactorily the above question, I have a right to say: It is all vain, baseless philosophy, and false wisdom. You speak through pure Reason, and claim to create a priori cognitions, inasmuch as you pretend not merely to dissect given conceptions but new connections which do not rest on the principle of contradiction, and which you think you conceive quite independently of all experience. How do you arrive at them, and how will you justify yourself in such pretensions? To appeal to the concurrence of the general common sense of mankind you cannot be allowed, for that is a witness whose reputation rests only on vulgar report.

    But indispensable as is the answer to this question, it is at the same time no less difficult, and although the chief cause why men have not long ago endeavoured to answer it, lies in the fact of its never having occurred to them that anything of the kind could be asked; there is a second cause, in that the satisfactory answer to this one question demands a more persistent, a deeper and more laborious reflection than the most diffuse work, on metaphysics, the first appearance of which has given promise of immortal fame to its author. And every thoughtful reader, on attentively considering the requirement of this problem, frightened at the outset by its difficulty, would regard it as insoluble; and indeed, were it not for the actual existence of such pure synthetic cognitions a priori, as altogether impossible. This happened in the case of David Hume, although he did not place the problem before him in such generality by far as is here done, and as must be done if the answer is to be decisive for the whole of metaphysics. For how is it possible, said the acute man, that when a conception is given me, I can pass out of it, and connect it with another, which is not contained in the former, and indeed in such a manner as if it necessarily belonged to it? Only experience can present us with such connections (this he concluded from the difficulty which he mistook for an impossibility), and all this imagined necessity, or, what is the same thing, knowledge assumed to be a priori, is nothing but a long habit of finding something true, and thence of holding the subjective necessity for objective. If the reader complains of the difficulty and trouble I shall give him in the solution of this problem, let him only set about the attempt to solve it in an easier way. He will then perhaps feel obliged to one who has undertaken for him the labour of such deep research, and rather show some surprise at the facility with which the solution has been able to be given, when the nature of the subject is taken into account. It has cost years of trouble to solve this problem in its whole universality (in the sense in which mathematicians use this word, namely, as sufficient for all cases), and to be able finally to present it in analytic form, such as the reader will here find.

    All metaphysicians are therefore solemnly and lawfully suspended from their occupations, till they shall have adequately answered the question—How are synthetic cognitions a priori possible? for in their answer alone consists the credentials they must produce, if they have aught to bring us in the name of pure Reason; in default of this, they can expect nothing else, than to be rejected, without any further inquiry as to their productions, by sensible people who have been so often deceived.

    If, on the other hand, they carry on their business not as a science, but as an art of wholesome persuasion, suitable to the general common sense of mankind, this calling cannot in fairness be denied them. In that case they will only use the modest language of a rational belief; they will admit that it is not allowed them even to conjecture, much less to know, anything, respecting that which lies beyond the boundaries of all possible experience, but merely to assume (not indeed for speculative use, for this they must renounce, but for purely practical purposes) what is possible and even indispensable for the direction of the understanding and will, in life. In this way alone can they possibly carry the reputation of wise and useful men, and they will do so the more in proportion as they renounce that of metaphysicians. For the object of the latter is to be speculative philosophers, and inasmuch as when we are concerned with judgments a priori, bare probabilities are not to be relied on (for what on its assumption is known a priori, is thereby announced as necessary), it cannot be allowed them to play with conjectures, but their assertions must be either science, or they are nothing at all.

    It may be said that the whole transcendental philosophy which necessarily precedes all metaphysics is itself nothing more than the full solution in systematic order and completeness of the question here propounded, and that therefore as yet we have no transcendental philosophy. For what bears its name is properly a part of metaphysics, but the former science must first constitute the possibility of the latter, and must therefore precede all metaphysics. Considering, then, that a complete and in itself entirely new science, and one respecting which no aid is to be derived from other sciences, is necessary before a single question can be adequately answered, it is not to be wondered at if the solution of the same is attended with trouble and difficulty, and even perhaps with some degree of obscurity.

    As we now proceed to this solution according to analytic method, in which we presuppose that such cognitions from pure Reason are real, we can only call to our aid two sciences of theoretic knowledge (with which alone we are here concerned), namely, pure mathematics and pure natural science, for only these can present to us objects in intuition, and therefore (if a cognition a priori should occur in them) show their truth or agreement with the object in concreto, i.e., their reality; from which to the ground of their possibility we can proceed on the analytic road. This facilitates the matter very much, as the universal considerations are not merely applied to facts but even start from them, rather than as in synthetic procedure, being obliged to be derived, wholly in abstracto, from conceptions.

    But from these real and at the same time well-grounded pure cognitions a priori, to rise to a possible one such as we are seeking, namely, to metaphysics as a science, we must needs embrace under our main question that which occasions it, to wit, the naturally given, though as regards its truth not unsuspicious, knowledge a priori lying at its foundation, and the working out of which, without any critical examination of its possibility, is now usually called metaphysics—in a word, the natural tendency to such a science; and thus the transcendental main question, divided into four other questions, will be answered step by step:

    1. How is pure mathematics possible?
    2. How is pure natural science possible?
    3. How is metaphysics in general possible?
    4. How is metaphysics as a science possible?

    It will be seen, that although the solution of these problems is chiefly meant to illustrate the essential contents of the Critique, it has nevertheless something special, which is of itself worthy of attention, namely, to seek the sources of given sciences in Reason, in order to investigate and measure this, their faculty of knowing something a priori, by means of the act itself. In this way the particular science itself must gain, if not in respect of its content, at least as regards its right employment, and while it throws light on the higher question of its common origin, at the same time give occasion to better elucidating its own nature.

    Section Six: The Main Transcendental Question—First Part: How is pure Mathematics possible?

    Here is a great and established branch of knowledge, already of remarkable compass, and promising unbounded extension in the future, carrying with it a thorough apodictic certainty, i.e., absolute necessity, and thus resting on no empirical grounds, but being a pure product of Reason, besides thoroughly synthetic. “How is it possible for the human Reason to bring about such a branch of knowledge entirely a priori?” Does not this capacity, as it does not and cannot stand on experience, presuppose some ground of knowledge a priori, lying deep-hidden, but which might reveal itself through these its effects, if their first beginnings were only diligently searched for?

    Section Seven: The First Part, Continued

    But we find that all mathematical knowledge has this speciality, that it must present its conception previously in intuition, and indeed a priori, that is, in an intuition that is not empirical but pure, without which means it cannot make a single step; its judgments therefore are always intuitive, whereas philosophy must be satisfied with discursive judgments out of mere conceptions; for though it can explain its apodictic doctrines by intuition, these can never be derived from such a source. This observation respecting the nature of mathematics, itself furnishes us with a guide as to the first and foremost condition of its possibility, namely, that some pure intuition must be at its foundation, wherein it can present all its conceptions in concreto and a priori at the same time, or as it is termed, construct them. If we can find out this pure intuition together with its possibility, it will be readily explicable how synthetic propositions a priori are possible in pure mathematics, and therefore, also, how this science is itself possible. For just as empirical intuition enables us, without difficulty, to extend synthetically in experience the conception we form of an object of intuition, by new predicates, themselves afforded us by intuition, so will the pure intuition, only with this difference: that in the last case the synthetic judgment a priori is certain and apodictic, while in the first case it is no more than a posteriori and empirically certain, because the latter only contains what is met with in chance empirical intuition, but the former what is necessarily met with in the pure intuition, inasmuch as being intuition a priori, it is indissolubly bound up with the conception before all experience or perception of individual things.

    Section Eight: The First Part, Continued

    But the difficulty seems rather to increase than to diminish by this step. For the question is now: How is it possible to intuit anything a priori? Intuition is a presentation, as it would immediately depend on the presence of the object. It seems therefore impossible to intuit originally a priori, because the intuition must then take place without either a previous or present object to which it could refer, and hence could not be intuition. Conceptions are indeed of a nature that some of them, namely, those containing only the thought of an object in general, may be very well formed a priori, without our being in immediate relation to the object (e.g., the conceptions of quantity, of cause, &c.), but even these require a certain use in concreto, i.e., an application to some intuition, if they are to acquire sense and meaning, whereby an object of them is to be given us. But how can intuition of an object precede the object itself?

    Section Nine: The First Part, Continued

    Were our intuition of such a nature as to present things as they are in themselves, no intuition a priori would take place at all, but it would always be empirical. For what is contained in the object in itself, I can only know when it is given and present to me. It is surely then inconceivable how the intuition of a present thing should enable me to know it as it is in itself, seeing that its properties cannot pass over into my presentative faculty. But granting the possibility of this, the said intuition would not take place a priori, that is, before the object was presented to me, for without it no ground of connection between my presentation and the object could be imagined; in which case it must rest on inspiration (Eingebung). Hence there is only one way possible, by which my intuition can precede the reality of the object and take place as knowledge a priori, and that is, if it contain nothing else but that form of sensibility which precedes in my subject all real impressions, by which I am affected by objects. For, that objects of sense can only be intuited in accordance with this form of sensibility, is a fact I can know a priori. From this it follows, that propositions merely concerning the form of sensible intuition, will be valid and possible for all objects of sense; and conversely, that intuitions possible a priori, can never concern other things than objects of our sense.


    1. What would go wrong if the objects of synthetic a priori knowledge (propositions like ‘the shortest distance between two points is a straight line’) were supposed to tell us about things as they are in themselves?
    2. What then do such propositions tell us about?

    Section Ten: The First Part, Continued

    Hence, it is only by means of the form of sensuous intuition that we can intuit things a priori, but in this way we intuit the objects only as they appear to our senses, not as they may be in themselves; an assumption absolutely necessary if synthetic propositions a priori are to be admitted as possible, or in the event of their being actually met with, if their possibility is to be conceived and defined beforehand.

    Now, such intuitions are space and time, and these lie at the basis of all the cognitions and judgments of pure mathematics, exhibiting themselves at once as apodictic and necessary. For mathematics must present all its conceptions primarily in intuition, and pure mathematics in pure intuition, i.e., it must construct them. For without this it is impossible to make a single step, so long, that is to say, as a pure intuition is wanting, in which alone the matter of synthetic judgments a priori can be given; because it cannot proceed analytically, that is, by the dissection of conceptions, but is obliged to proceed synthetically. The pure intuition of space constitutes the basis of geometry—even arithmetic brings about its numerical conceptions by the successive addition of units in time; but above all, pure mechanics can evolve its conception of motion solely with the aid of the presentation of time. Both presentations, however, are mere intuitions; for when all that is empirical, namely, that belongs to feeling, is left out of the empirical intuitions of bodies and their changes (motion), space and time still remain over, and are therefore pure intuitions, lying a priori at the foundation of the former. For this reason, they can never be left out, but being pure intuitions a priori, prove that they are the bare forms of our sensibility, which must precede all empirical intuition, i.e., the perception of real objects, and in accordance with which objects can be known a priori, though only as they appear to us.

    Section Eleven: The First Part, Continued

    The problem of the present section is therefore solved. Pure mathematics is only possible as synthetic knowledge a priori, in so far as it refers simply to objects of sense, whose empirical intuition has for its foundation a pure intuition a priori (that of time and space), which intuition is able to serve as a foundation, because it is nothing more than the pure form of sensibility itself, that precedes the real appearance of objects, in that it makes them in the first place possible. Yet this faculty of intuiting a priori does not concern the matter of the phenomenon, i.e., that which is feeling (Empfindung) in the latter, for this constitutes the empirical element therein; but only its form, space and time. Should anybody cast the least doubt on the fact that neither of them are conditions of things in themselves, but only dependent on their relation to sensibility. I should be glad to be informed how he deems it possible to know a priori, and therefore before all acquaintance with the things, that is, before they are given us, how their intuition must be constructed, as is here the case with space and time. Yet this is quite conceivable, as soon as they both count for nothing more than formal determinations of our sensibility, and the objects merely as phenomena, for in that case the form of the phenomenon, that is, the pure intuition, can be conceived as coming from ourselves, in other words, as a priori.


    1. What makes mathematics possible? How can we anticipate geometrical features of all objects of possible experience before we experience them?

    Section Twelve: The First Part, Continued

    To contribute something to the explanation and confirmation of the above, we have only to consider the ordinary and necessary procedure of geometricians. All the proofs of complete likeness between two given figures, turn at last upon the fact of their covering each other; in other words, of the possibility of substituting one, in every point, for the other, which is obviously nothing else but a synthetic proposition resting on immediate intuition. Now this intuition must be given pure and a priori, for otherwise the proposition in question could not count as apodictically certain, but would possess only empirical certainty. We could only say in that case, it has been always so observed, or it is valid so far as our perception has hitherto extended. That complete space, itself no boundary of a further space, has three dimensions, and that no space can have more than this number, is founded on the proposition that not more than three lines can bisect each other at right angles in a single point. But this proposition cannot be presented from conceptions, but rests immediately on intuition, and indeed on pure a priori intuition, because it is apodictically certain that we can require a line to be drawn out to infinity (in indefinitum), or that a series of changes (e.g., spaces passed through by motion) shall be continued to infinity, and this presupposes a presentation of space and time, merely dependent on intuition, namely, so far as in itself, it is bounded by nothing, for from conceptions it could never be concluded. Pure intuitions a priori, then, really lie at the foundation of mathematics, and these make its synthetic and apodictically valid propositions possible, and hence our transcendental deduction of conceptions in space and time explains at the same time the possibility of pure mathematics, which without such a deduction, and without our assuming that “all which can be given to our senses (the outer in space, the inner in time) is only intuited by us, as it appears to us, and not as it is in itself,” might indeed be conceded, but could in nowise be understood.

    Section Thirteen: The First Part, Continued

    Those who are unable to free themselves from the notion, that space and time are real qualities (Beschaffenheiten) appertaining to the things in themselves, may exercise their wits on the following paradoxes, and when they have in vain attempted their solution, may suppose, being freed from their prejudices at least for a few moments, that perhaps the degradation of space and time to the position of mere forms of our sensible intuition, may have some foundation.

    When two things are exactly alike [equal] in all points that can be cognised in each by itself (i.e., in all respecting quantity or quality), it must follow, that one can in all cases and relations be put in the place of the other, without this substitution occasioning the least cognisable difference. This indeed applies to plane figures in geometry; but there are many spherical figures, which in spite of this complete internal agreement exhibit in their external relations an agreement falling short of admitting one to be put in the place of the other.

    For instance, two spherical triangles on opposite hemispheres, having an arc of the equator as a common base, are perfectly equal both in respect of their sides and their angles, so that in neither of them, if separately and at the same time completely described, would anything be found which was not equally present in the other; and yet notwithstanding this, one cannot be put in the place of the other, i.e., on the opposite hemisphere, and herein consists the internal difference of both triangles, that no understanding can indicate as internal, but which reveals itself only by means of the external relation in space. I will now adduce some more ordinary cases taken from common life.

    What can more resemble my hand or my ear, and be in all points more like, than its image in the looking-glass? And yet I cannot put such a hand as I see in the glass in the place of its original; for when the latter is a right hand, the one in the glass is a left hand, and the image of the right ear is a left one, which can never take the place of the former. Now, here there are no internal differences that could be imagined by any understanding. And yet the differences are internal, so far as the senses teach us, for the left hand cannot, despite all equality and similarity, be enclosed within the same bounds as the right (they are not congruent); the glove of one hand cannot be used for the other. What then is the solution? These objects are not presentations of things as they are in themselves, and as the pure understanding would cognise them, but they are sensuous intuitions, i.e., phenomena, the possibility of which rests on the relations of certain unknown things in themselves to something else, namely, to our sensibility. Now, space is the form of the outward intuition of these, and the inward determination of every space is only possible through the determination of outward relations to the whole space, of which each [separate] space is a part (i.e., by its relation to the outward sense); in other words, the part is only possible through the whole, which though it could never be the case with things in themselves, namely, with objects of the mere understanding, can very well be so with mere phenomena. Hence we can render the difference of similar and equal, though incongruent things (e.g., spirals winding opposite ways) intelligible by no single conception, but only by the relation of the right and left hands, which refers immediately to intuition.


    1. Kant here appeals to ‘incongruent counterparts’—objects that are qualitatively identical in all their internal relations (in the case of the hands, the relation between thumb and forefinger, say) and yet cannot be substituted one for another. Can you think of other pairs of incongruent counterparts?

    Kant goes on in the Prolegomena to answer some objections (see his three ‘Remarks’ below). But before turning to objections, we should look at Kant’s arguments for his position from the Critique. In this selection, from the ‘Transcendental Aesthetic’ of the CPR, Kant lays out four arguments about space.

    The CPR: “The Transcendental Aesthetic”, Section One, “Of Space”

    1. Space1 is not a conception which has been derived from outward experiences. For, in order that certain sensations may relate to something without me (that is, to something which occupies a different part of space from that in which I am); in like manner, in order that I may represent them not merely as without, of, and near to each other, but also in separate places, the representation of space must already exist as a foundation. Consequently, the representation of space cannot be borrowed from the relations of external phenomena through experience; but, on the contrary, this external experience is itself only possible through the said antecedent representation.
    2. Space then is a necessary representation a priori, which serves for the foundation of all external intuitions. We never can imagine or make a representation to ourselves of the non-existence of space, though we may easily enough think that no objects are found in it. It must, therefore, be considered as the condition of the possibility of phenomena, and by no means as a determination dependent on them, and is a representation a priori, which necessarily supplies the basis for external phenomena.
    3. Space is no discursive, or as we say, general conception of the relations of things, but a pure intuition. For, in the first place, we can only represent to ourselves one space, and, when we talk of divers spaces, we mean only parts of one and the same space. Moreover, these parts cannot antecede this one all-embracing space, as the component parts from which the aggregate can be made up, but can be cogitated only as existing in it. Space is essentially one, and multiplicity in it, consequently the general notion of spaces, of this or that space, depends solely upon limitations. Hence it follows that an a priori intuition (which is not empirical) lies at the root of all our conceptions of space. Thus, moreover, the principles of geometry—for example, that “in a triangle, two sides together are greater than the third,” are never deduced from general conceptions of line and triangle, but from intuition, and this a priori, with apodictic certainty.
    4. Space is represented as an infinite given quantity. Now every conception must indeed be considered as a representation which is contained in an infinite multitude of different possible representations, which, therefore, comprises these under itself; but no conception, as such, can be so conceived, as if it contained within itself an infinite multitude of representations. Nevertheless, space is so conceived of, for all parts of space are equally capable of being produced to infinity. Consequently, the original representation of space is an intuition a priori, and not a conception.

    1. Kant thinks you can mount exactly parallel arguments for time. Choose one of the four arguments above and re-cast it as an argument about the nature of time.

    The Prolegomena, Remark One

    Pure mathematics, and especially pure geometry, can only possess objective reality under the condition that they merely refer to objects of sense, in view of which, however, the axiom holds good that our sensuous presentation is in nowise a presentation of things in themselves, but only of the manner wherein they appear to us. Hence it follows that the propositions of geometry are not the mere determinations of a creation of our poetic fancy, which therefore cannot be referred with confidence to real objects, but that they are necessarily valid of space, and consequently of everything that may be found in space; because space is nothing more than the form of all external phenomena, under which alone objects of sense can be given us. Sensibility, the form of which lies at the foundation of geometry, is that whereon the possibility of external phenomena rests; so these can never contain anything but what geometry prescribes for them. It would be quite different if the senses had to present the objects as they are in themselves. For in that case it would by no means follow from the presentation of space (which the geometrician posits with all its properties as an a priori basis), that all this, together with what is deduced therefrom, is exactly so constituted in Nature. The space of the geometrician would be regarded as a mere fiction, and no objective validity ascribed to it, because we do not see why things must necessarily conform to the image that we make of them spontaneously and beforehand. But when this image, or rather this formal intuition, is the essential property of our sensibility by means of which alone objects are presented to us; and yet this sensibility presents not things in themselves, but only their appearances, it is quite easy to conceive, and at the same time incontrovertibly proved, that all the external objects of our sense-world must necessarily conform with the most complete accuracy to the propositions of geometry. For sensibility, by its form of external intuition (space) with which the geometrician is occupied, makes those objects themselves (though as mere appearances) primarily possible. It will always remain a remarkable phenomenon in the history of philosophy that there has been a time when even mathematicians who were also philosophers began to doubt, not indeed of the correctness of their propositions in so far as they concerned space, but of the objective validity and application of this conception, with all its geometrical determinations, to Nature. They were concerned lest a line in Nature might consist of physical points, and the true space in the object, accordingly of simple parts, whereas the space the geometrician has in his mind can never consist of such. They did not recognise that this space in thought makes the physical space, i.e., the extension of matter, itself possible; that the latter is no quality of things in themselves, but only a form of our sensible faculty of presentation; that all objects in space are mere phenomena, i.e., are not things in themselves, but presentations of our sensuous intuition; and hence that space, as the geometrician thinks it, is exactly the form of sensuous intuition we find a priori in ourselves, containing the ground of possibility of all external phenomena (as regards their form); and that these must necessarily and in the most exact manner agree with the propositions of the geometrician, which he draws from no fictitious conception, but from the subjective foundation of all external phenomena, namely, the sensibility itself. In such and no other manner can the geometrician be ensured as to the indubitable objective reality of his propositions against all the cavils of an arid metaphysics, however strange it may seem to him, owing to his not having reverted to the sources of his conceptions.

    The Prolegomena, Remark Two

    All that is given us as object, must be given us in intuition. But all our intuition takes place by means of the senses alone; the understanding intuits nothing, but only reflects. Inasmuch then as the senses, according to what is above observed, never enable us to cognise, not even in one single point, the things in themselves, but only their phenomena, while these are mere presentations of sensibility, “all bodies, together with the space in which they are found, must be held to be nothing but mere presentations, existing nowhere but in our thoughts.” Now is this not the plainest idealism?

    Idealism consists in the assertion that there exist none but thinking entities; the other things we think we perceive in intuition, being only presentations of the thinking entity, to which no object outside the latter can be found to correspond. I say, on the contrary, things are given as objects discoverable by our senses, external to us, but of what they may be in themselves we know nothing; we know only their phenomena, i.e., the presentations they produce in us as they affect our senses. I therefore certainly admit that there are bodies outside us, that is, things, which although they are wholly unknown to us, as to what they may be in themselves, we cognise through presentations, obtained by means of their influence on our sensibility. To these we give the designation of body, a word signifying merely the phenomenon of that to us unknown, but not the less real, object. Can this be termed idealism? It is indeed rather the contrary thereof.

    That without calling in question the existence of external things, it may be said of a number of their predicates that they do not belong to the things in themselves, but only to their phenomena, and have no self-existence outside our presentation, is what had been generally accepted and admitted long before Locke’s time, but more than ever since then. To these belong heat, colour, taste, &c. No one can adduce the least ground for saying that it is inadmissible on my part, when for important reasons I count in addition the remaining qualities of bodies called primarias, such as extension, place, and more especially space, together with what is dependent thereon (impenetrability or materiality, figure, &c.) amongst the number of these phenomena. And just as little as the man who will not admit colours to be properties of the object in itself, but only to pertain as modifications to the sense of sight, is on that account called an idealist, so little can my conception be termed idealistic because I find in addition that all properties which make up the intuition of a body belong merely to its appearance. For the existence of a thing, which appears, is not thereby abolished as with real idealism, but it is only shown that we cannot cognise it, as it is in itself, through the senses.

    I should like to know how my assertions must be fashioned, if they are not to contain an idealism. I should doubtless have to say, that the presentation of space is not alone completely in accordance with the relation of our sensibility to objects, for that I have already said, but that it is exactly similar to the object itself; an assertion to which no sense can be attached, just as little as that the feeling of red has a similarity with the cinnabar producing this feeling in me.


    1. One of the early criticisms of the CPR was that Kant’s view is really just a complicated version of immaterialism; Kant was called ‘a Teutonic Berkeley.’ why is Kant so sure that his view about space and time—that they are empirically real (i.e., characteristics of all possible objects of experience) but transcendentally ideal (i.e., they do not characterize things as they are in themselves)—does not amount to Berkeleyan idealism?

    The Prolegomena, Remark Three

    Hence we may readily set aside an easily foreseen but pointless objection: namely, that through the ideality of space and time, the whole sense-world would be changed to sheer illusion. All philosophical insight into the nature of sensuous cognition was ruined from the first by making sensibility to consist simply in a confused mode of presentation, by which we cognise the things as they are, without having the capacity to bring everything in this, our cognition, to clear consciousness. On the other hand, it has been proved by us that sensibility does not consist in this logical distinction of clearness and obscurity, but in the genetic distinction of the origin of knowledge itself, since sensuous cognition does not present the things as they are, but only the manner in which they affect our senses; and that therefore through them mere phenomena, and not the things themselves, are given to the understanding for reflection. After this necessary correction, a consideration presents itself, arising from an inexcusable and almost purposeless misapplication, as though my doctrine changed all the objects of sense into mere illusion.

    When an appearance is given us we are quite free as to what we thence infer with regard to the matter. The former, namely, the appearance, rests on the senses, but the judgment on the understanding; and the only question is, whether or not there is truth in the determination of the object. But the distinction between truth and dream is not decided by the construction of the presentations, which are referred to objects, for they are alike in both, but by the connection of the same according to the rules determining the coherence of presentations in the conception of an object, and by whether they can stand together in an experience or not. Hence the fault does not lie with the phenomena, if our cognition takes the illusion for truth, i.e., if an intuition, whereby an object is given, is held to be the conception of the object or its existence, which the understanding alone can cogitate. The senses present to us the course of the planets as first forwards and then backwards, and in this there is neither falsehood nor truth, because so long as it is considered as an appearance only, no judgment is yet formed as to the objective character of their motion. But inasmuch as when the understanding does not take great care lest this subjective mode of presentation be held for objective, a false judgment may easily arise; it is said, they seem to go back; the illusion, however, is not to be laid to the account of the senses, but of the understanding, whose province alone it is to form an objective judgment on the phenomenon.

    In this manner, even if we did not reflect on the origin of our presentations, and let our intuitions of sense contain what they may, if it be but connected according to the coherence of all knowledge in an experience, [we shall find that] deceptive illusion or truth will arise according as we are negligent or careful; for it concerns solely the use of sensuous presentations in the understanding, and not their origin. In the same way, if I hold all presentations of sense together with their form, namely, space and time, to be nothing but phenomena, and the latter to be a mere form of sensibility not present in the objects external to it, and I make use of these presentations only in reference to a possible experience, there is not therein the least temptation to error, neither is there an illusion implied in my regarding them as mere appearances; for in spite of this they can rightly cohere according to the rules of truth in an experience. In such wise all the propositions of geometry respecting space are valid just as much of all the objects of sense, and therefore in respect of all possible experience, whether I regard space as a mere form of sensibility or as something inhering in the things themselves. But in the first case alone can I conceive how it is possible to know a priori the above propositions concerning objects of external intuition. Otherwise everything remains in respect to all merely possible experience just as though I had never undertaken this departure from the popular judgment.

    But, let me only venture with my conceptions of space and time beyond all possible experience, which is unavoidable if I give them out as qualities appertaining to the things in themselves (for what should prevent me from assuming them as valid of these same things, even though my senses were differently constructed, and whether they were suited to them or not?) then a serious error may arise, resting on an illusion giving out as universally valid what is a mere condition of the intuition of things pertaining to my subject (certain for all the objects of sense, and thereby for all possible experience), because I refer them to things in themselves and fail to limit them to the conditions of experience.

    So far, then, from my doctrine of the ideality of space and time reducing the whole sense-world to mere illusion, it is rather the only means of ensuring the application of some of the most important cognitions, namely, those propounded a priori by mathematics, to real objects, and of guarding them from being held as illusion. For without this observation it would be quite impossible to ascertain whether the intuitions of space and time we borrow from no experience, but which nevertheless lie a priori in our faculty of presentation, were not mere self-made cobwebs of the brain, to which no object, or at least no adequate object, corresponded, and geometry itself therefore a mere illusion; instead of which, its incontestable validity in respect of all objects of the sense-world, owing to these being simply phenomena, has been able to be demonstrated by us.

    Secondly, so far from my principles, because they reduce the presentations of the senses to phenomena, turning the truth of experience into illusion, they are rather the only means of guarding against the transcendental illusion, whereby metaphysics has always been deceived and misled into childish endeavours to grasp at soap-bubbles, by taking phenomena, which are mere presentations, for things in themselves; whence have resulted the remarkable assumptions of the antinomy of Reason, of which I shall make mention farther on, and which are abolished by the single observation that appearance, as long as it is used simply in experience, produces truth, but as soon as it passes beyond the bounds of the latter and becomes transcendent, nothing but pure illusion.

    Inasmuch, then, as I leave their reality to the things we intuit to ourselves through the senses, and only limit our sensuous intuition of those things in that they in no particular, not even in the pure intuitions of space and time, represent more than the appearance of the above things, and never their constitution as they are in themselves; this is no thorough-going illusion of my own invention [applied to] Nature. My protestation against all supposition of an idealism is so decisive and clear, that it might seem superfluous were it not for incompetent judges, who like to have an old name for every departure from their distorted although common opinion, and who never judge of the spirit of philosophical terminology, but cling simply to the letter, being ready to put their own delusion in the place of well-defined perceptions, and so to distort and deform them. For the fact of my having myself given my theory the name of transcendental idealism, can justify no one in confounding it with the idealism of Descartes (though this was only a problem, on account of whose insolubility every one was free, in the opinion of Descartes, to deny the existence of the bodily world, because it could never be satisfactorily solved), or with the mystical and visionary idealism of Berkeley, against which and other similar cobwebs of the brain our Critique rather contains the best specific. For what is by me termed idealism, does not touch the existence of things (the doubt of the same being what properly constitutes idealism in the opposite sense), for to doubt them has never entered my head, but simply concerns the sensuous presentation of things, to which space and time chiefly belong; and of these and of all phenomena I have only shown that they are neither things (but only modes of presentation), nor determinations belonging to things in themselves. But the word ‘transcendental’, which with me never implies a reference to our knowledge of things, but only to our faculty of knowledge (Erkenntnissvermogen) should guard against this misconception. Rather, however, than occasion its further continuance, I prefer to withdraw the expression, and let it be known as critical (idealism). If it be indeed an objectionable idealism, to change into mere presentations real things (not phenomena), what name shall be applied to that which conversely turns mere presentations into things? I think we may term it the dreaming idealism, in contradistinction to the foregoing, that may be termed the visionary, but both of which ought to have been obviated by my elsewhere so-called transcendental, but better, critical, idealism.


    1. One way previous philosophers (like Locke and Descartes) made sense of the appearance/reality distinction was in terms of those representations that correspond to things in themselves and those that don’t. Why isn’t Kant entitled to make the distinction in this way?
    2. How, then, does he propose to make the distinction?

    The Prolegomena, Section Fourteen: The Main Transcendental Question—Second Part: How is pure Natural Science possible?

    Nature is the existence of things, in so far as it is determined according to universal laws. If Nature signified the existence of things in themselves, we could never know it either a priori or a posteriori. Not a priori, for how shall we know what applies to things in themselves? since this can never be done by the dissection of our conceptions (analytic propositions). For what I want to know, is not what is contained in my conception of a thing (for that concerns its logical nature), but what in the reality of the thing is superadded to this conception, by which the thing itself is determined outside my conception. My understanding and the conditions under which alone it can connect the determination of things in their existence, prescribes no rules for the things in themselves; these do not conform themselves to my understanding, but my understanding conforms itself to them. They must therefore be previously given me, in order for these determinations to be discovered in them; and in this case they would not be known a priori.

    But a posteriori such a knowledge of the nature of things in themselves would be equally impossible. For if experience is to teach me laws to which the existence of things is subordinated, these must, in so far as they concern things in themselves, of necessity also apply to them outside my experience. Now experience teaches me, indeed, what exists and how it exists, but never that it exists necessarily in such a manner and no other. It can never, therefore, teach the nature of things in themselves.

    Section Fifteen: The Second Part, Continued

    We are nevertheless really in possession of a pure natural science, which a priori and with all the necessity requisite to apodictic propositions, puts forward laws to which Nature is subordinated. I only require here to call to witness that propaedeutic, which, under the title of universal natural science, precedes all physical science based on empirical principles. Therein we find mathematics applied to phenomena, also those discursive principles (from conceptions) constituting the philosophical part of pure natural knowledge. But the latter also contains much that is not pure, and independent of the sources of experience, as the conception of motion, of impenetrability (on which the empirical conception of matter rests), of inertia and others, which prevent its being called a perfectly pure natural science. Besides, it is only concerned with the objects of the external sense, and thus furnishes no example of a pure natural science in its strictest meaning; for this would have to bring Nature generally under universal laws, irrespective of whether it concerned the object of the outer or of the inner sense of physical science, or of psychology. But among the principles of the above universal physical science are to be found some that really possess the universality we require, as the proposition that substance continues and is permanent, and that all which happens is at all times previously determined by a cause, according to fixed laws. These are really universal natural laws, existing completely a priori. There is then in fact a pure natural science, and now the question arises—how is it possible?

    Section Sixteen: The Second Part, Continued

    The word Nature further assumes another meaning, which defines the object, whereas in the above meaning the mere regularity of the existence of the determinations of things generally, is denoted. Nature considered materialiter is the sum-total of all the objects of experience. With this we are alone concerned at present, for things which could never be objects of an experience were they to be known according to their nature, would necessitate us to form conceptions, to which meaning could never be given in concreto (in any example from a possible experience), and of the nature of which we should be obliged to make conceptions alone, whose reality, that is, whether they really referred to objects or were mere figments of thought, could never be decided. With that which cannot be an object of experience, the knowledge of which would be hyperphysical, or anything like it, we have here nothing at all to do, but only with the natural knowledge whose reality can be confirmed by experience, notwithstanding its being a priori possible, and preceding all experience.

    Section Seventeen: The Second Part, Continued

    The formal in Nature, in this narrower signification, is then the regularity of all the objects of experience, and in so far as they are known a priori, their necessary regularity. But it has been just demonstrated that the laws of Nature can never be known a priori in objects, in so far as they are considered not as the objects of a possible experience but as things in themselves. We are not here concerned with things in themselves (the qualities of which we put on one side), but merely with things as the objects of a possible experience, and the sum-total of which is properly what we call Nature. And I now ask, whether, if the question be as to the possibility of a cognition of Nature a priori, it would be better to formulate the problem, as follows: How is it possible to cognise a priori the necessary regularity of things as objects of experience? or, How is the necessary regularity of experience itself in respect of all its objects, generally [possible to be cognised a priori]?

    Seen in its true light, the solution of the problem, whether presented in the one or in the other form, in respect of the pure cognition of Nature (which constitutes the real point of the question) is in the end altogether the same. For the subjective laws under which alone an experiential cognition of things is possible, are valid also of those things as objects of a possible experience (though not indeed as things in themselves; but the latter we are not here considering). It is quite the same, then, whether I say: Without the law—that on an event being perceived, it must invariably be referred to something preceding it, upon which it follows according to a universal rule—a judgment of perception can never avail as experience; or whether I express myself thus: Everything that experience teaches us, happens, must have a cause.

    It is, however, advisable to choose the first formula. For as we can have a knowledge a priori and before all given objects, of those conditions under which alone an experience in respect of them is possible, but never of what laws, they, without reference, to a possible experience, are subordinated to, in themselves; we shall not be able to study the nature of things a priori, otherwise than by investigating the conditions and universal (although subjective) laws, under which such a knowledge is alone possible (in respect of mere form), as experience, and in accordance therewith determine the possibility of things as objects of experience. Were I to choose the second mode of expression and seek the conditions a priori under which Nature is possible as an object of experience, I should easily be led into misunderstanding, and fancy I had to explain Nature as a thing in itself, and I should then be fruitlessly involved in endless endeavours to seek laws for things of which nothing is given me.

    We shall here, therefore, be simply concerned with experience, and the universal and a priori given conditions of its possibility, and thence determine Nature as the complete object of all possible experience. I think it will be understood, that I do not refer to the rules for the observation of a nature already given, which presuppose experience, or how through experience we can arrive at the laws of Nature, for these would not then be laws a priori, and would give no pure science of Nature; but how the conditions a priori of the possibility of experience are at the same time the sources from which all the universal laws of Nature must be derived.

    Section Eighteen: The Second Part, Continued

    We must first of all observe then, that, although all the judgments of experience are empirical, i.e., have their ground in the immediate perception of sense, yet on the other hand all empirical judgments are not judgments of experience, but that beyond the empirical, and beyond the given sensuous intuition generally, special conceptions must be superadded, having their origin entirely a priori in the pure understanding, under which every perception is primarily subsumed, and by means of which only it can be transformed into experience.

    Empirical judgments, in so far as they have objective validity, are judgments of experience; but those which are merely subjectively valid I call judgments of perception. The last require no pure conception of the understanding; but only the logical connection of perception in a thinking subject. But the first demand, above the presentations of sensuous intuition, special conceptions originally generated in the understanding, which make the judgment of experience objectively valid.

    All our judgments are at first mere judgments of perception; they are valid simply for us, namely, for our subject. It is only subsequently that we give them a new reference, namely, to an object, and insist that they shall be valid for us always, as well as for every one else. For when a judgment coincides with an object, all judgments must both coincide with the same object and with one another, and thus the objective validity of the judgment of experience implies nothing more than the necessary universal validity of the same. But, on the other hand, when we see reason to hold a judgment of necessity universally valid (which never hinges on the perception itself, but on the pure conception of the understanding under which the perception is subsumed), we are obliged to regard it as objective, i.e., as expressing not merely the reference of the perception to a subject but a quality of the object; for there would be no reason why the judgments of other persons must necessarily coincide with mine, if it were not that the unity of the object to which they all refer, and with which they coincide, necessitates them all agreeing with one another.

    Section Nineteen: The Second Part, Continued

    Objective validity and necessary universality (for every one) are therefore exchangeable notions, and although we do not know the object in itself, yet when we regard a judgment as at once universal and necessary, objective validity is therewith understood. We cognise in this judgment the object (though it remain unknown what it is in itself) by the universal and necessary connection of given perceptions, and as this is the case with all objects of sense, judgments of experience owe their objective validity not to the immediate cognition of the object (for this is impossible), but merely to the condition of universality in the empirical judgment, which, as has been said, never rests on empirical, or on any sensuous conditions, but on a pure conception of the understanding. The object in itself always remains unknown; but when through the conception of the understanding, the connection of the presentations given to our sensibility by the latter is determined as universally valid, the object is determined by this relation, and the judgment is objective.

    That the room is warm, the sugar sweet, the wormwood bitter, are merely subjectively valid judgments. I do not expect that I shall always, or that every other person, will find them as I do now. They only express a reference of two sensations to the same subject, namely, myself, and that only in my present state of perception, and are not therefore valid of objects. I call these judgments of perception. With judgments of experience the case is altogether different. What experience teaches me under certain circumstances, it must teach me at all times, and every other person as well; its validity is not limited to the subject or to the state of the latter at a particular time. I pronounce, therefore, all such judgments to be objectively valid. For instance when I say “the air is elastic”, this judgment is immediately a judgment of perception, since I only refer the feelings in my senses to one another. If I insist it shall be called a judgment of experience, I expect this connection to stand under a condition making it universally valid. I insist, that is, that I at all times and every other person, shall necessarily so combine the same perceptions, under the same circumstances.


    1. A key distinction is between judgments of perception (JOPs) and judgments of experience (JOEs). Judgments are propositions in which the mind unites intuitions according to a rule (see below). What is the distinction between JOPs and JOEs? what transforms a mere JOP into a JOE?

    Section Twenty: The Second Part, Continued

    We must therefore dissect experience, in order to see what is contained in this product of sense and understanding, and how the judgment of experience itself is possible. The intuition of which I am conscious, namely, perception, which merely belongs to the senses, lies at its foundation. But secondly, judgment (which pertains solely to the understanding) also belongs to it. This [act of] judgment may be twofold; firstly, I may simply compare the perceptions in a particular state of my own consciousness; or secondly, I may combine them in a consciousness in general. The first judgment is a simple judgment of perception, and has therefore only subjective validity, being the mere connection of perceptions in my mental state, without reference to the object. Hence it is not sufficient for experience, as is commonly imagined, to compare perceptions and to connect them in a consciousness by means of the judgment. No universality and necessity in the judgment can arise therefrom, by means of which alone it can be objectively valid, and experience.

    There is another and quite a different judgment presupposed, before perception can become experience. The given intuition must be subsumed under a conception determining the form of the judgment generally in respect of the intuition, connecting the empirical consciousness of the last in a consciousness in general, and thereby obtaining universality for the empirical judgment; such a conception is a pure a priori conception of the understanding, that does nothing but determine for an intuition the general manner in which it can serve for judgment. Should the conception be that of cause, it determines the intuition subsumed under it in respect of judgment generally; for instance, in the case of air, that in respect of expansion, it stands in the relation of antecedent to consequent, in a hypothetical judgment. The conception of cause is then a pure conception of the understanding, entirely distinct from all possible perception, and only serves to determine that presentation contained under it, in respect of judgment generally, in short, to make a universally valid judgment possible.

    Now, before a judgment of perception can become a judgment of experience, it is first of all necessary that the perception be subsumed under these conceptions of the understanding. For instance, air belongs to the conception of causes, which determines the judgment regarding its extension, as hypothetical. In this way, the extension is represented not merely as belonging to my perception of air in my particular state, or in many of my states, or in a particular state of the perception of others, but as necessarily belonging thereto; and the judgment, “the air is elastic”, becomes universally valid, and therefore a judgment of experience, preceded by certain judgments, which subsume the intuition of air under the conception of cause and effect, and thereby the perceptions, not merely with respect to one another in my subject, but relatively to the form of judgment generally (here the hypothetical), and thus make the empirical judgment universally valid. …

    Section Twenty-one: The Second Part, Continued

    In order to demonstrate the possibility of experience, in so far as it rests on pure a priori conceptions of the understanding, we must first present what belongs to judgment generally, and the various momenta of the understanding in the same, in a complete table, for the pure conceptions of the understanding, which are nothing more than conceptions of intuitions in general, in so far as these are determined in themselves by one or other of these momenta of judgment, that is, are necessarily and universally valid, must run exactly parallel to them [viz., these momenta]. In this way, the axioms a priori of the possibility of all experience as an objectively valid empirical cognition, are precisely determined. For they are nothing but propositions, subsuming all perception (in accordance with certain universal conditions of perception), under the above pure conceptions of the understanding.

    Logical Table of the Judgements

    Quantity Quality Relation Modality
    Universal Affirmative Categorical Problematical
    Particular Negative Hypothetical Assertorical
    Singular Infinite Disjunctive Apodictic

    Transcendental Table of the Concepts of the Understanding

    Quantity Quality Relation Modality
    Unity (the measure) Reality Substance Possibility
    Plurality (the amount) Negation Cause Actuality
    Totality (the whole) Limitation Reciprocity Necessity

    Section Twenty-one (A): The Second Part, Continued

    In order to grasp the preceding in a single notion, it is necessary to remind the reader that we are not here speaking of the origin of experience, but of that which lies within it. The first belongs to empirical psychology, and would exist without the second, which belongs to the critique of cognition, and especially to that of the understanding, and can never be sufficiently developed.

    Experience consists of intuitions, belonging to sensibility, and of judgments which are entirely the work of the understanding. But the judgments the understanding constructs merely out of sensuous intuitions, are not, by far, judgments of experience. For in the one case the judgment simply connects the perceptions, as they are given in sensuous intuition; but in the other, the judgments must say what experience generally contains, and not what the mere perception, the validity of which is purely subjective, contains. The judgment of experience must add something to a judgment, over and above the sensuous intuition, and the logical connection of the same (after it has been made universal by comparison), something that determines the synthetic judgment, as at once necessary and thereby universally valid; and this can be nothing else but that conception which presents the intuition as determined in itself, in respect to one form of judgment rather than another, i.e., a conception of that synthetic unity of intuitions, which can only be presented through a given logical function of the judgment.

    Section Twenty-two: The Second Part, Continued

    The sum of the above is this: the business of the senses is to intuit, that of the understanding to think. But to think is to unite presentations in a consciousness. This union is either merely relative to the subject, and is contingent and subjective, or is given unconditionally, and is necessary or objective. The union of presentations in a consciousness is judgment. Thinking, then, is the same as judging, or referring presentations to judgments in general. Hence judgments are either entirely subjective when presentations are solely referred to a consciousness in one subject, and are therein united, or they are objective when they are united in a consciousness in general, that is, are necessarily united therein. The logical momenta of all judgments are so many possible modes of uniting presentations in a consciousness. But if they serve as conceptions, they are conceptions of the necessary union of the same in a consciousness, and therefore principles of objectively valid judgments. This union in a consciousness is either analytic by identity, or synthetic by the combination and addition of different presentations to one another. Experience consists in the synthetic connection of phenomena (perceptions) in a consciousness, in so far as this is necessary. Hence pure conceptions of the understanding are those under which all perceptions must be previously subsumed, before they can serve as judgments of experience, in which the synthetic unity of perceptions is presented as necessary and universal.


    Before going forward, it’s important to make sure we have the basic terminology in hand:

    Sensibility
    The faculty of the mind that ‘receives’ intuitions
    Intuition (Anschauung)
    A bit of sensory awareness. This can refer to any kind of sensory awareness, from any sense modality whatsoever, as well as those bits of awareness that are also called up by the memory.

    All intuitions are in time (as objects of thought, they always occur in temporal positions relative to each other); some of these are also in space (that is, they represent their objects as having spatial location). Time is the form of inner intuition; space, outer intuition.

    The matter of an intuition is its purely sensory element—in visual images, for example, this would include color. Its form is the spatial or temporal structure the sensibility imposes on this ‘matter.’ So even in sensation the mind is not purely passive—it is actively structuring its perceptions.

    Understanding
    The faculty that unites intuitions into thoughts

    Sensibility generates intuitions; the understanding generates concepts. But how can we bring these together? Intuitions without concepts are blind; concepts without intuitions are empty. How are the two going to be united?

    Judgment
    The basic unit of thought, in which the mind unites distinct intuitions into a coherent whole. All judgments are judgments of perception; some judgments are also judgments of experience. The basic unit of thought, as opposed to sensibility, then, is the proposition: a claim that things are thus-and-so.

    A judgment of perception unites intuitions merely according to the logical forms of judgment (page 600, Section 21). These judgments only have subjective validity; that is, they make a claim only about the perceptions of the person doing the judging. In a JOP, I “merely compare perceptions and connect them in a consciousness of my state” (599).

    Example: when I say, ‘if I perceive gold being immersed in aqua regia, I then perceive its dissolution,’ I have not said anything about whether this must happen, will happen in all cases, or, most importantly, why it happens. I have only made a report about my own perceptions. Or: suppose I conjure up an image of a dagger. I can unite these intuitions according to the mere form of judgment and bring together the perceptions of the handle and the blade. But I don’t think of the dagger as a real object that others would see if they were here and could use to open their mail.

    A judgment of experience, by contrast, unites intuitions according to the logical forms of judgment and the pure concepts of the understanding, the categories. When I say that putting gold in aqua regia causes it to dissolve, I am not merely saying that the one perception follows the other. I am saying that this happens necessarily, i.e., that it happens according to a law of nature. My claim is objectively valid: that is, I am claiming that anyone similarly situated will observe exactly what I observe when I dropped the gold into the solution. (Objective validity is a necessary, but not sufficient, condition for truth. An objectively valid judgment is a candidate for being true; it is not automatically so. See the CPR, A58/B83).

    “Experience consists in the synthetic connection of appearances in consciousness, so far as this connection is necessary.” Thinking of this connection as necessary and as holding for all similarly situated cognitive beings requires the categories, pure concepts of the understanding that structure our intuitions into experience. Concepts of any kind are always rules for making judgments. The concept dog, for example, is a rule that tells you which of your intuitions to unite so you can come to think of those intuitions as a single thing, viz., a dog. So the categories are the maximally general rules we use to generate experience. (Imagine what your ‘experience’ would be like if you lacked these pure concepts.)

    Sensibility imposes a form on the material element of sensation and generates an intuition; in parallel fashion, the understanding imposes a form on intuitions and generates experience.

    Section 22 above presents a clear summary of Kant’s argument so far. Here is one way to sketch it; I encourage you to try your own.

    1. We have experience (in the weighty sense), not just a flood of intuitions (what sensibility provides) and not just a set of thoughts about our mental states (JOPs).
    2. Experience is possible only if we unite intuitions in propositions.
    3. Only judgment can unite these intuitions.
    4. To do more than think about our own mental states (JOPs), we have to use certain concepts.
    5. These concepts allow us to unite our intuitions in propositions that, if true at all, are true for all possible subjects of experience. They involve a necessary connection between the objects of my intuitions, not merely the connection of intuitions in my own private experience.
    6. These concepts must be synthetic and a priori.

    Section Twenty-three: The Second Part, Continued

    Judgments, considered merely as the union of given presentations in a consciousness, are rules. These rules, in so far as they present the union as necessary, are rules a priori, and in so far as there are none beyond them from which they can be derived, they are axioms. Since, then, in respect of the possibility of all experience, when viewed as the mere form of thought, there are no conditions of the judgments of experience beyond those which bring the phenomena in the various forms of their intuition under the pure conceptions of the understanding which make the empirical judgment objectively valid, these must be the a priori axioms of all possible experience.

    The axioms of possible experience are at the same time the universal laws of Nature as known a priori. And thus the problem contained in our present second question—How is pure natural science possible? is solved. For the systematic character required by the form of a science is met with here in completeness, since beyond the above-named formal conditions of all judgments in general, that is, of all the general rules to be found in logic, there are none possible, and these constitute a logical system; while the conceptions founded upon them, containing the conditions a priori of all synthetic and necessary judgments, [constitute] in the same way a transcendental system, and finally the axioms, by means of which all phenomena are subsumed under these conceptions, [constitute] a physiological system, i.e., a system of nature, preceding all empirical knowledge of nature, rendering this in the first place possible, and therefore to be properly termed the universal and pure natural science. …

    Section Twenty-five: The Second Part, Continued

    As regards the relation of phenomena, and indeed simply as to their existence, the determination of this relation is not mathematic but dynamic, and can never be valid objectively, and therefore adequate to an experience, if it be not subordinated to principles a priori rendering the cognition of experience regarding them in the first place possible. Hence phenomena must be subsumed under the conception of substance, which lies at the foundation of all determination of existence as a conception of the thing itself; or secondly, in so far as a succession, that is, an event, is met with among the phenomena, under the conception of an effect in reference to cause; or in so far as co-existence is to be cognised objectively, that is, through a judgment of experience, under the conception of community (reciprocal action); and these principles a priori lie at the foundation of objectively valid although empirical judgments, that is, the possibility of experience in so far as it is to connect the existence of objects in Nature. These principles are the particular laws of Nature, which may be termed dynamic.

    There belongs, finally, to the judgments of experience the cognition of the agreement and connection, not so much of phenomena among one another in experience, as of their relation to experience generally, which unites either their agreement with the formal conditions cognised by the understanding or their coherence with the material of sense and of perception, or both, in one conception, and consequently contains possibility, reality and necessity, according to universal natural laws, thereby constituting the physiological doctrine of method, the distinction between truth and hypotheses, and the limits of the reliability of the latter.

    Section Twenty-six: The Second Part, Continued

    Although the third table of the principles drawn from the nature of the understanding on the critical method, shows a completeness in itself, which raises it far above every other that has been vainly attempted or may be attempted in the future [to be drawn] from the nature of the thing itself, in a dogmatic way, inasmuch as therein all synthetic axioms a priori have been produced in accordance with a principle, that is, the possibility of judgment in general, which constitutes the essence of experience, in reference to the understanding, in such a manner that one may be certain there are no more such axioms (a satisfaction never to be obtained from the dogmatic method)—yet this is by far not its greatest service.

    Attention must be paid to the ground of proof, which discovers the possibility of this knowledge a priori, and limits at the same time all such axioms by a condition, that must never be overlooked, if they are not to be misunderstood, and extended farther in use than the original sense attached to them by the understanding will admit of: namely, that they only contain the conditions of possible experience in general, in so far as it is subordinated to laws a priori. Thus I do not say that things in themselves contain a quantity, their reality, a degree, their existence, connection of accidents in a substance, &c.; for this no one can prove, because such a synthetic connection is simply impossible out of mere conceptions, where all reference to sensuous intuition on the one hand, and all connection of the same in a possible experience on the other, is wanting. The essential limitation of conceptions in these axioms is, therefore, that all things only stand under the above-mentioned conditions a priori as objects of experience.

    From this there follows, in the second place, a special and peculiar mode of proof of the foregoing: that the axioms in question do not refer directly to phenomena and their relation, but to the possibility of experience of which phenomena constitute the matter but not the form, i.e., to objective and universally valid synthetic propositions, wherein judgments of experience are distinguished from mere judgments of perception. This happens in that the phenomena as mere intuitions, taking in a portion of space and time, are subordinated to the conception of quantity, which unites the manifold in the same synthetically in accordance with a priori rules; and that in so far as the perception contains feeling as well as intuition, between which and zero, namely, its total disappearance, a progression by diminution always takes place, the real of the phenomena must have a basis, seeing that in itself it takes in no portion of space or time. But this progression towards it [viz., reality] from empty time or space, is only possible in time. Consequently, although feeling as the quality of empirical intuition can never be known a priori in respect of that wherein it is specifically distinguished from other feelings, it can nevertheless be distinguished in a possible experience generally, as quantity of perception intensively [distinct] from every other of the same kind; which means the application of mathematics to Nature in respect of the sensuous intuition, by which the former is given us, and by which it becomes in the first place possible and definite.

    But the reader must give the greatest attention to the mode of proof of the principles coming under the name of analogies of experience. For inasmuch as these do not, like the principles of the application of mathematics to natural science generally, concern the generation of intuitions, but the connection of their existence in an experience, this can be nothing but the determination of existence in time according to necessary laws, under which alone they are objectively valid, and therefore experience. Thus the proof of synthetic unity does not turn on the connection of things in themselves, but of perceptions, and even of these, not in respect of their content but of their determination in time, and of the relation of existence thereto, according to universal laws. These universal laws contain, therefore, the necessity of the determination of existence in time generally (consequently, according to a rule of the understanding, a priori) when the empirical determination in the relative time is to be objectively valid, that is, experience. …

    Section Twenty-seven: The Second Part, Continued

    It is here the place to raze Hume’s doubt from its foundation. He maintained justly that we can in nowise discern through Reason the possibility of causation, namely, the reference of the existence of one thing to the existence of some other thing posited by the former. I may add to this, that we can just as little discern the conception of subsistence, i.e., the necessity contained therein, that a subject must lie at the basis of the existence of a thing, and itself be no predicate of any other thing. [I would say even] that we can form no conception of the possibility of such a thing (though we can point out examples of its use in experience). In the same way this inconceivability attaches even to the community of things, since it is not discernible how, from the state of one thing, a consequence can be drawn as to the state of some totally different thing, external to it, and vice versa; and how substances of which each has its own separate existence, are necessarily dependent on one another. At the same time, I am far from regarding these conceptions as merely borrowed from experience, and the necessity, that is presented in them, as fictitious and mere illusion, induced in us by long custom. I have, rather, sufficiently shown that both they and the axioms deduced from them, subsist a priori before all experience, and possess indubitable objective correctness, though unquestionably only in respect of experiences.

    Section Twenty-eight: The Second Part, Continued

    Although I cannot have the slightest notion of such a connection of things in themselves as of their existing as substances, working as causes, or being able to stand in community with other [substances] as parts of a real whole, I can still less conceive such properties in phenomena as phenomena, because these conceptions contain nothing that lies in the phenomena, but something the understanding alone can conceive. We have, then, from such a connection of presentations in our understanding, and, indeed, in judgments generally, a similar conception, namely, that presentations cohere in one kind of judgments, as subject with reference to predicate, in another as cause with reference to effect, in a third as parts together making up a complete possible cognition. Further, we cognise a priori, that without the presentation of an object, in respect of one or the other of these momenta, to be considered as something definite, we could have no cognition that could be valid of objects, and if we occupied ourselves with the object in itself, there would be no single mark possible, by which I could cognise whether it was determined in respect of one or of another cogitated moment, i.e., whether it cohered under the conception of substance, or of cause, or (in relation to other substances) of reciprocity, for of the possibility of such a connection of existence I should have no conception. But it is not the question, how things in themselves, but how cognition of experience of things in respect of cogitated momenta of judgments generally, is defined, that is, how things as objects of experience can and should be subsumed under the above conceptions of the understanding. And hence it is clear, that I fully recognise not only the possibility, but also the necessity, of subsuming all phenomena under these conceptions, namely, of using them as axioms of the possibility of experience.

    Section Twenty-nine: The Second Part, Continued

    Let us now attempt a solution of Hume’s problematical conception, namely, the conception of Cause. Firstly, there is given me, a priori, by means of Logic, the form of a conditioned judgment generally, one cognition as antecedent and another as consequent. But it is possible that in the perception, a rule of the relation may be met with, which will say, that on [the occurrence of a] given phenomenon another always follows (though not conversely), and this would be a case in which to make use of the hypothetical judgment, and to say, for instance, if a body be illumined long enough by the sun, it will become warm. There is certainly no necessity of connection here, in other words, no conception of cause. But I continue: if the above proposition, which is a mere subjective connection of perception, is to be a proposition of experience, it must be regarded as necessary and universally valid; but such a proposition would run: Sun is through its light the cause of heat. The above empirical rule is now looked upon as law, and indeed, not alone as valid of phenomena, but valid of them in relation to a possible experience, which requires thoroughly, and therefore necessarily, valid rules. I perfectly understand, then, the conception of Cause, as a conception necessarily belonging to the mere form of experience, and its possibility as a synthetic union of perceptions, in a consciousness in general; but the possibility of a thing in general as a cause I do not understand, because the conception of cause does not refer at all to things, but only indicates the condition attaching to experience, namely, that this can be only an objectively valid knowledge of phenomena, and their sequence in time, in so far as the antecedent can be united to the consequent according to the rule of hypothetical judgments.


    1. What is Kant’s solution to Hume’s doubts about the concept of cause?
    2. In the terms we used to describe Hume’s own solution to this nexus of problems, is Kant’s solution a straightforward solution, a skeptical solution, or something else? Why?

    Section Thirty: The Second Part, Continued

    Hence the pure conceptions of the understanding have no meaning whatever, when they quit the objects of experience and refer to things in themselves (noumena). They serve, as it were, to spell out phenomena, that these may be able to be read as experience. The axioms arising from their relation to the world of sense, only serve our understanding for use in experience. Beyond this, are only arbitrary combinations, destitute of objective reality, and the possibility of which can neither be known a priori, nor their reference to objects be confirmed, or even made intelligible by an example, because all examples are borrowed from some possible experience, and consequently the objects of those conceptions are nothing but what may be met with in a possible experience.

    This complete solution of Hume’s problem, although it turns out to be contrary to the opinion of its originator, preserves for the pure conceptions of the understanding their origin a priori, and for the universal laws of Nature their validity as laws of the understanding, but in such a manner that their use is limited to experience, because their possibility has its basis, solely, in the reference of the understanding to experience; not because they are derived from experience, but because experience is derived from them, which completely reversed mode of connection never occurred to Hume.

    The following result of all previous researches follows from the above investigations: “All synthetic axioms a priori are nothing more than principles of possible experience,” and can never be referred to things in themselves, but only to phenomena as objects of experience. Hence pure mathematics no less than pure natural science can never refer to anything more than mere phenomena, and only present that which either makes experience in general possible, or which, inasmuch as it is derived from these principles, must always be able to be presented in some possible experience.


    1. What constraint does Kant put on the application of the categories?

    Since Kant’s reply to Hume is at the heart of his work, we should look at the argument he mounts against Hume in the CPR.

    The CPR: “The Second Analogy”

    Principle of the Succession of Time According to the Law of Causality: All changes take place according to the law of the connection of Cause and Effect. 2

    Proof … I perceive that phenomena succeed one another, that is to say, a state of things exists at one time, the opposite of which existed in a former state. In this case, then, I really connect together two perceptions in time. Now connection is not an operation of mere sense and intuition, but is the product of a synthetical faculty of imagination, which determines the internal sense in respect of a relation of time. But imagination can connect these two states in two ways, so that either the one or the other may antecede in time; for time in itself cannot be an object of perception, and what in an object precedes and what follows cannot be empirically determined in relation to it. I am only conscious, then, that my imagination places one state before and the other after; not that the one state antecedes the other in the object. In other words, the objective relation of the successive phenomena remains quite undetermined by means of mere perception. Now in order that this relation may be cognized as determined, the relation between the two states must be so cogitated that it is thereby determined as necessary, which of them must be placed before and which after, and not conversely. But the conception which carries with it a necessity of synthetical unity, can be none other than a pure conception of the understanding which does not lie in mere perception; and in this case it is the conception of “the relation of cause and effect,” the former of which determines the latter in time, as its necessary consequence, and not as something which might possibly antecede (or which might in some cases not be perceived to follow). It follows that it is only because we subject the sequence of phenomena, and consequently all change, to the law of causality, that experience itself, that is, empirical cognition of phenomena, becomes possible; and consequently, that phenomena themselves, as objects of experience, are possible only by virtue of this law.

    Our apprehension of the manifold of phenomena is always successive. The representations of parts succeed one another. Whether they succeed one another in the object also, is a second point for reflection, which was not contained in the former. Now we may certainly give the name of object to everything, even to every representation, so far as we are conscious thereof; but what this word may mean in the case of phenomena, not merely in so far as they (as representations) are objects, but only in so far as they indicate an object, is a question requiring deeper consideration. In so far as they, regarded merely as representations, are at the same time objects of consciousness, they are not to be distinguished from apprehension, that is, reception into the synthesis of imagination, and we must therefore say: “The manifold of phenomena is always produced successively in the mind.” If phenomena were things in themselves, no man would be able to conjecture from the succession of our representations how this manifold is connected in the object; for we have to do only with our representations. How things may be in themselves, without regard to the representations through which they affect us, is utterly beyond the sphere of our cognition. Now although phenomena are not things in themselves, and are nevertheless the only thing given to us to be cognized, it is my duty to show what sort of connection in time belongs to the manifold in phenomena themselves, while the representation of this manifold in apprehension is always successive. For example, the apprehension of the manifold in the phenomenon of a house which stands before me, is successive. Now comes the question whether the manifold of this house is in itself successive—which no one will be at all willing to grant. But, so soon as I raise my conception of an object to the transcendental signification thereof, I find that the house is not a thing in itself, but only a phenomenon, that is, a representation, the transcendental object of which remains utterly unknown. What then am I to understand by the question: “How can the manifold be connected in the phenomenon itself—not considered as a thing in itself, but merely as a phenomenon?” Here that which lies in my successive apprehension is regarded as representation, whilst the phenomenon which is given me, notwithstanding that it is nothing more than a complex of these representations, is regarded as the object thereof, with which my conception, drawn from the representations of apprehension, must harmonize. It is very soon seen that, as accordance of the cognition with its object constitutes truth, the question now before us can only relate to the formal conditions of empirical truth; and that the phenomenon, in opposition to the representations of apprehension, can only be distinguished therefrom as the object of them, if it is subject to a rule which distinguishes it from every other apprehension, and which renders necessary a mode of connection of the manifold. That in the phenomenon which contains the condition of this necessary rule of apprehension, is the object.

    Let us now proceed to our task. That something happens, that is to say, that something or some state exists which before was not, cannot be empirically perceived, unless a phenomenon precedes, which does not contain in itself this state. For a reality which should follow upon a void time, in other words, a beginning, which no state of things precedes, can just as little be apprehended as the void time itself. Every apprehension of an event is therefore a perception which follows upon another perception. But as this is the case with all synthesis of apprehension, as I have shown above in the example of a house, my apprehension of an event is not yet sufficiently distinguished from other apprehensions. But I remark also that if in a phenomenon which contains an occurrence, I call the antecedent state of my perception, A, and the following state, B, the perception B can only follow A in apprehension, and the perception A cannot follow B, but only precede it. For example, I see a ship float down the stream of a river. My perception of its place lower down follows upon my perception of its place higher up the course of the river, and it is impossible that, in the apprehension of this phenomenon, the vessel should be perceived first below and afterwards higher up the stream. Here, therefore, the order in the sequence of perceptions in apprehension is determined; and by this order apprehension is regulated. In the former example, my perceptions in the apprehension of a house might begin at the roof and end at the foundation, or vice versa; or I might apprehend the manifold in this empirical intuition, by going from left to right, and from right to left. Accordingly, in the series of these perceptions, there was no determined order, which necessitated my beginning at a certain point, in order empirically to connect the manifold. But this rule is always to be met with in the perception of that which happens, and it makes the order of the successive perceptions in the apprehension of such a phenomenon necessary.

    I must, therefore, in the present case, deduce the subjective sequence of apprehension from the objective sequence of phenomena, for otherwise the former is quite undetermined, and one phenomenon is not distinguishable from another. The former alone proves nothing as to the connection of the manifold in an object, for it is quite arbitrary. The latter must consist in the order of the manifold in a phenomenon, according to which order the apprehension of one thing (that which happens) follows that of another thing (which precedes), in conformity with a rule. In this way alone can I be authorized to say of the phenomenon itself, and not merely of my own apprehension, that a certain order or sequence is to be found therein. That is, in other words, I cannot arrange my apprehension otherwise than in this order.

    In conformity with this rule, then, it is necessary that in that which antecedes an event there be found the condition of a rule, according to which in this event follows always and necessarily; but I cannot reverse this and go back from the event, and determine (by apprehension) that which antecedes it. For no phenomenon goes back from the succeeding point of time to the preceding point, although it does certainly relate to a preceding point of time; from a given time, on the other hand, there is always a necessary progression to the determined succeeding time. Therefore, because there certainly is something that follows, I must of necessity connect it with something else, which antecedes, and upon which it follows, in conformity with a rule, that is necessarily, so that the event, as conditioned, affords certain indication of a condition, and this condition determines the event.

    Let us suppose that nothing precedes an event, upon which this event must follow in conformity with a rule. All sequence of perception would then exist only in apprehension, that is to say, would be merely subjective, and it could not thereby be objectively determined what thing ought to precede, and what ought to follow in perception. In such a case, we should have nothing but a play of representations, which would possess no application to any object. That is to say, it would not be possible through perception to distinguish one phenomenon from another, as regards relations of time; because the succession in the act of apprehension would always be of the same sort, and therefore there would be nothing in the phenomenon to determine the succession, and to render a certain sequence objectively necessary. And, in this case, I cannot say that two states in a phenomenon follow one upon the other, but only that one apprehension follows upon another. But this is merely subjective, and does not determine an object, and consequently cannot be held to be cognition of an object—not even in the phenomenal world.

    Accordingly, when we know in experience that something happens, we always presuppose that something precedes, whereupon it follows in conformity with a rule. For otherwise I could not say of the object that it follows; because the mere succession in my apprehension, if it be not determined by a rule in relation to something preceding, does not authorize succession in the object. Only, therefore, in reference to a rule, according to which phenomena are determined in their sequence, that is, as they happen, by the preceding state, can I make my subjective synthesis (of apprehension) objective, and it is only under this presupposition that even the experience of an event is possible.


    There are two alternatives to consider: rationalism and empiricism. But the rationalist picture is hard to take seriously, since it amounts to a deus ex machina. Let’s look at empiricism.

    Recall Locke on power (Essay, II.xxi). For Locke, we first notice changes in the world and then infer that bodies have powers that bring them about. (Cp. Locke on space: we experience space and derive our idea from that experience.) So for Locke, there’s a primitive difference in experience between objects and events, between things and the changes we observe. Let’s see if this makes any sense.

    Consider two sets of appearances: (O) those that we take to be of a house, and (E) those we take to be of a log being consumed by fire. Is the difference between these something we, as it were, just see? Is it there to be discovered in experience?

    What will Kant say? Why? (See 689.)

    Moral: a succession of appearances is not necessarily an appearance of succession.

    So: the difference between O and E cannot lie in the appearances themselves. In imagination, I can order the successive appearances of the house (O) any way I want to, and still have a representation of a house. I don’t think of parts of the house happening one after another. But I do think that the successive appearances of the log in (E) must be ordered in exactly one way. Another way to put this: in O, I don’t think that anyone observing the house has to have exactly the same order of appearances. But in E, I do: I think that anyone observing the event fire has to see the appearances in the same order I do. Where does this must come from?

    It can only derive from the fact that I am subjecting the appearances in (E) to a rule, which I am not applying to (O).

    1. This rule cannot be derived from experience; in order to have the experience of (E), I first have to deploy the rule. What is that rule?

    So again: where Locke thought we have observation, Kant thinks we have invention.

    We can now say that every event must have a cause. But notice that we’ve proved it only for ______.


    We now return to the Prolegomena. Having established the necessary application of the Categories to experience, Kant goes on to discuss the origin of metaphysical illusion. Why are we not content to admit that the Categories apply only to objects as we must experience them, not to things in themselves?

    The Prolegomena, Section Thirty-one: The Second Part, Continued

    And thus we have at last something definite to hold by in all metaphysical undertakings, which hitherto, bold enough, but always blind, have pursued all things without distinction. Dogmatic thinkers have never let it occur to them, that the goal of their endeavours should be extended such a short way from them, and even those most confident in their imagined common sense have started with conceptions and principles of mere Reason, legitimate and natural, it is true, but intended merely for use in experience, [in search of] spheres of knowledge, for which they neither knew nor could know of any definite boundaries, because they had neither reflected nor could reflect on the nature or even the possibility of any such pure understanding.

    Many a naturalist of pure Reason (by which I understand he who ventures to decide in questions of metaphysics, without any science) might well profess that what has been here put forward with so much preparation, or if he will have it so, with tediously pedantic pomp, he has long ago not merely conjectured but known and penetrated, by the prophetic spirit of his common sense, namely, “that with all our Reason, we can never pass beyond the field of experiences.” But he must confess, notwithstanding, when questioned seriatim as to his principles of Reason, that amongst these there are many to be found not drawn from experience, and therefore valid, independently thereof, and a priori. How then, and on what grounds, will he hold the dogmatist and himself in limits, who use these conceptions and principles outside all possible experience, simply because they are recognised as independent of it? And even this adept of common sense, in spite of all his pretended, cheaply acquired, wisdom, is not proof against wandering, unobserved, beyond the objects of experience into the field of chimeras. He is, indeed, in the ordinary way, deeply enough involved therein, although by the use of popular language, by putting everything forward as probability, reasonable supposition or analogy, he gives some colour to his groundless assumptions.

    Section Thirty-two: The Second Part, Continued

    From the earliest ages of philosophy, investigators of pure Reason have postulated, beyond the sensible essences (phenomena) which constitute the world of sense, special essences of the understanding (noumena) which are supposed to constitute a world of understanding; and since they held appearance and illusion [Erscheinung und Schein] for the same thing, which in an undeveloped epoch is to be excused, ascribed reality to the intelligible essence alone.

    In fact, when we regard the objects of sense, as is correct, as mere appearances, we thereby at the same time confess that a thing in itself lies at their foundation, although we do not know it, as it is constituted in itself, but only its appearance, that is, the manner in which our senses are affected by this unknown something. The understanding then, by accepting appearances, admits also the existence of things in themselves, and we may even say that the presentation of such essences as lie at the basis of appearances, in short, mere essences of the understanding, is not only admissible, but unavoidable.

    Our critical deduction does not by any means exclude such things (noumena), but rather limits the principles of æsthetic, in so far that these should not be extended to all things, whereby everything would be changed into mere appearance, but that they should only be valid of objects of a possible experience. Essences of the understanding are hereby admitted only by the emphasising of this rule, which admits of no exception, that we know nothing definite whatever of these pure essences of the understanding, neither can we know anything of them, because our pure conceptions of the understanding no less than our pure intuitions, concern nothing but objects of a possible experience, in short, mere essences of sense, and as soon as we leave these, the above conceptions have not the least significance remaining.

    Section Thirty-three: The Second Part, Continued

    There is indeed something seductive about our pure conceptions of the understanding, as regards temptation to a transcendent use; for so I name that which transcends all possible experience. Not only do our conceptions of substance, force, action, reality, &c., which are entirely independent of experience containing no phenomenon of sense, really seem to concern things in themselves (noumena); but what strengthens this supposition is, that they contain a necessity of determination in themselves, to which experience can never approach. The conception of cause contains a rule, according to which from one state another follows in a necessary manner; but experience only teaches us that often, or at most usually, one state of a thing follows upon another, and can therefore acquire neither strict universality nor necessity.

    Hence these conceptions of the understanding seem to have far too much significance and content for mere use in experience to exhaust their entire determination, and the understanding builds in consequence, unobserved, by the side of the house of experience, a much more imposing wing, which it fills with sheer essences of thought, without even noticing that it has overstepped the legitimate bounds of its otherwise correct conceptions.

    Section Thirty-four: The Second Part, Continued

    There were two important, and indeed altogether indispensable, although exceedingly dry investigations necessary, that have been undertaken in the Critique, in the first of which it was shown that the senses do not furnish the pure conceptions of the understanding in concreto, but only the schema for their use, and that the object which conforms to it is only to be met with in experience as the [common] product of the understanding, and the materials of sense. In the second investigation it is shown, that—notwithstanding the independence of our pure conceptions of the understanding and principles of experience, even to the apparently greater range of their use—nothing whatever could be conceived through them outside the field of experience, because they can do nothing but determine the merely logical form of judgment in respect of given intuitions. But since, beyond the field of sensibility, no intuition is given, these pure conceptions become totally void of meaning, in as much as they can in no way be presented in concreto. Consequently, all these noumena together with their sum-total, an intelligible world, are nothing but presentations of a problem, the subject of which in itself is indeed possible, but the solution of which is, by the nature of our understanding, utterly impossible, since our understanding is no faculty of intuition, but is merely the connection of given intuitions in an experience, and must comprise therefore all objects for our conceptions; but apart from these, all conceptions which cannot be supported by an intuition, must be without meaning.

    Section Thirty-five: The Second Part, Continued

    The imagination may perhaps be forgiven, if it sometimes dreams, and fails to keep itself carefully within the limits of experience; for certainly it is invigorated and strengthened by a free flight like this, and it is always easier to moderate its boldness than to stimulate its languor. But for the understanding, which ought to think, to dream instead, can never be forgiven, as it is our only support in setting bounds to the fantasies of the imagination, where this is necessary.

    It begins, however, very innocently and modestly. First of all, it reduces the elementary cognitions inhering in it before all experience, but having their application, notwithstanding, in experience, to their pure state. Gradually it lets fall these limits; and what is there then to hinder it, seeing that the understanding has taken its principles quite freely from itself? First of all, it is led to newly invented powers in Nature, soon after to essences outside Nature, in a word, to a world for whose fitting-up we can never fail in material, because by a fruitful imagination this will always be richly procured, and although not substantiated by experience, will yet never be confuted by it. This is the reason why young thinkers are so fond of metaphysics, treated in a genuinely dogmatic manner, and sacrifice to it their time and talents which might be otherwise useful.

    But it is of no avail attempting to moderate these fruitless attempts of pure Reason, by all manner of cautions as to the difficulty of the solution of such deeply-hidden questions, lamentations over the limits of our Reason, and by lowering assertions to mere conjectures. For if their impossibility be not clearly shown, and the self-knowledge of Reason be not [raised to] a true science, in which the field of its right use is separated from that of its nugatory and fruitless use, so to speak, with geometrical certainty, these vain endeavours will never be completely laid aside.

    Section Thirty-six: How is Nature itself possible?

    This question, which is the highest point the transcendental philosophy can ever touch, and to which it must also, as its boundary and completion, be directed, properly comprises two questions.

    Firstly: How is Nature, in its material signification, namely, as intuition, as the sum-total of phenomena—how is space, time, and that which fills them both, namely, the object of feeling in general—possible? The answer is, by means of the construction of our sensibility, in accordance with which, it is affected in a special manner by objects, in themselves unknown and entirely distinct from these appearances. This answer has been given in the book itself in the Transcendental Æsthetic, but in these Prolegomena in the solution of the first general question.

    Secondly: How is Nature in its formal signification—as the sum-total of the rules to which all phenomena must be subordinated, if they are to be thought of as connected in an experience—possible? The answer can only be: It is only possible by means of the construction of our understanding, in accordance with which all the above presentations of sensibility are necessarily referred to a consciousness, and whereby the special manner of our thought (namely, by rules), and by means of these, experience (which is to be wholly distinguished from a knowledge of things in themselves) is possible. …

    But how this special property of our sensibility itself, or of our understanding together with the necessary apperception lying at its basis, and at that of all thought, is possible, will not admit of any further solution or answer, because we invariably require it for all answers and for all thought of objects.

    There are many laws of Nature that we can only know by means of experience, but regularity in the connection of phenomena, i.e., Nature in general, we can never learn through experience, because experience itself requires such laws, and these lie at the foundation of its possibility a priori. The possibility of experience in general is at once the universal law of Nature, and the axioms of the one are at the same time the laws of the other. For we know nothing of Nature otherwise than as the sum-total of phenomena, namely, of presentations in us, and hence can derive the law of their connection in no other way than from the principles of the same connection in ourselves; in other words, from the conditions of necessary union in a consciousness, which constitutes the possibility of experience.

    Even the main proposition, worked out through the whole of this section, that universal natural laws are to be known a priori, of itself leads to the further proposition, that the highest legislation of Nature must lie in ourselves, namely, in our understanding, and that we must seek its universal laws, not in Nature, by means of experience; but conversely, must seek Nature, as to its universal regularity, solely in the conditions of the possibility of experience lying in our sensibility and understanding. For how would it otherwise be possible to know these laws a priori if they be not rules of analytic knowledge, but actually synthetic extensions of the same? Such a necessary agreement of the principles of possible experience with the laws of the possibility of Nature can only occur from one of two causes; either the laws are borrowed from Nature by means of experience, or conversely, Nature is derived from the laws of the possibility of experience generally, and is entirely the same thing as the purely formal regularity of the latter. The first supposition contradicts itself, for the universal laws of Nature can and must be known a priori (i.e., independently of all experience), and be posited as the basis of the empirical use of the understanding; so that only the second [hypothesis] remains to us.

    But we must distinguish the empirical laws of Nature, which always presuppose particular perceptions, from the pure or universal natural laws, which without any particular perceptions at their foundation, merely contain the conditions of their necessary union in an experience; and in respect of the last, Nature and possible experience are the same thing. Hence, as in this, the legitimacy rests on the necessary connection of phenomena in an experience, in other words, on the original laws of the understanding (without which we could cognise no object of the sensuous world whatever), it sounds at first singular, but is none the less certain, when I say in respect of the latter: The understanding draws its laws (a priori) not from Nature, but prescribes them to it. …


    How is Natural Science possible?

    1. Step One: Clarifying the problem

      Natural science, despite appearances, includes a ‘pure’ (i.e., non-empirical) element. Every science makes two assumptions. What are they?

      These assumptions are pre-conditions for even engaging in science in the first place.

      Two main competitors: rationalism and (Lockean) empiricism. Neither works. So we need to start over and find a third way. Turn now to JOEs. What makes them possible?

      But these just are the universal laws of nature (602). That is, the principles upon which all sciences depend also lie at the bottom of all _________, and make it possible.

      So we’ve really got just one problem here: how can we justify claims that involve the pure concepts of the understanding, claims like cause, effect, substance, etc.?

    2. Step Two: What did Hume teach us?
      1. These claims cannot be empirical. Why?
      2. But nor can they be analytic. Why?

      These are the prongs of Hume’s fork; for him, every proposition is either a matter of fact or a relation of ideas. Hume concludes that these principles have only __________ necessity; they are the ‘bastard offspring’ of __________ impregnated by _________.

    3. Step Three: Putting it all together:

      Hume is right to claim that experience can’t justify __________. But he’s wrong to think that this means that ____________________. Instead, the concept of cause and effect is a pure ____________________. That is, it is not something we read off from experience, but something we use to construct experience in the first place. There’s no room for skepticism about causation, since to have any experiences at all, I must be uniting __________ in judgments according to the concept of a cause. Like geometry, pure natural science depends on _____________________________.

    4. Step Four: What are the consequences?

      But just as with geometry, this means that the categories can be applied ____________________. Is Kant saying that things as they are in themselves must be connected by cause and effect, or that they are things at all, in the way a tree is a thing? __________, because ___________________.

    5. Another way to see the same point: remember, all judgments require ____________________. Applied to things in themselves, the categories have __________. (See Sections Thirty and Thirty-four). When we try to apply the categories to things in themselves, we leave out _________, and so generate only empty logical forms, structure without stuffing.

    So far, we’ve seen Kant explain how metaphysics’ companions in guilt—pure natural science and mathematics—are entitled to use synthetic a priori reasoning. Before we see Kant close the circle and return to metaphysics proper, we should take a look at an important passage from the CPR that tells us a great deal about the limitations Kant is imposing on the application of the categories, and on the kinds of questions we can and cannot ask.

    The CPR, “Deduction of the Pure Concepts of the Understanding”

    This cognition,3 which is limited to objects of experience, is not for that reason derived entirely from experience. There are, unquestionably, elements of cognition, which exist in the mind a priori [e.g., the Categories and the pure forms of intuition]. Now there are only two ways in which a necessary harmony of experience with the conceptions of its objects can be cogitated. Either experience makes these conceptions possible, or the conceptions make experience possible. The former of these statements will not hold good with respect to the categories (nor in regard to pure sensuous intuition), for they are a priori conceptions, and therefore independent of experience. …

    Consequently, nothing remains but to adopt the second alternative, namely, that on the part of the understanding the categories do contain the grounds of the possibility of all experience. But with respect to the questions how they make experience possible, and what are the principles of the possibility thereof with which they present us in their application to phenomena, the following section on the transcendental exercise of the faculty of judgement will inform the reader.

    It is quite possible that someone may propose a species of preformation-system of pure reason—a middle way between the two—to wit, that the categories are neither innate and first a priori principles of cognition, nor derived from experience, but are merely subjective aptitudes for thought implanted in us contemporaneously with our existence, which were so ordered and disposed by our Creator, that their exercise perfectly harmonizes with the laws of nature which regulate experience. Now, not to mention that with such an hypothesis it is impossible to say at what point we must stop in the employment of predetermined aptitudes, the fact that the categories would in this case entirely lose that character of necessity which is essentially involved in the very conception of them, is a conclusive objection to it. The conception of cause, for example, which expresses the necessity of an effect under a presupposed condition, would be false, if it rested only upon such an arbitrary subjective necessity of uniting certain empirical representations according to such a rule of relation. I could not then say, “The effect is connected with its cause in the object (that is, necessarily),” but only, “I am so constituted that I can think this representation as so connected, and not otherwise.” Now this is just what the sceptic wants. For in this case, all our knowledge, depending on the supposed objective validity of our judgement, is nothing but mere illusion; nor would there be wanting people who would deny any such subjective necessity in respect to themselves, though they must feel it. At all events, we could not dispute with any one on that which merely depends on the manner in which his subject is organized.


    1. The problem Kant confronts here is this: why is it that our concepts match experience? Kant says, it’s because we construct our experiences. One alternative is empiricism (experience gives us the concepts). What’s wrong with this view?
    2. What is the third alternative here? Does it remind you of any view we’ve encountered before?

    Now we return to the Prolegomena, and Kant’s answer to the main question of that work.

    The Prolegomena, Section Forty: The Main Transcendental Question—Third Part: How is Metaphysics possible at all?

    Pure mathematics and pure natural science would not require for their own security and certainty a deduction such as we have just concluded with respect to them both; for the former rests upon its own evidence, while the latter, although arising from the pure sources of the understanding, is dependent upon the complete substantiation of experience, a witness it is unable altogether to repudiate and do without, seeing that with all its certainty, as philosophy, it can never compete with mathematics. Both these sciences required the foregoing investigation, not for their own sake, but for the sake of another science, namely, metaphysics.

    Metaphysics is concerned not merely with natural conceptions, having invariably an application in experience, but, in addition to these, with pure conceptions of Reason, which can never be given in any possible experience; that is, with conceptions whose objective reality (as distinguished from simple cobwebs of the brain), and with assumptions whose truth or falsity can be confirmed or discovered by no experience. This part of metaphysics is precisely that which constitutes its essential purpose, all else being merely a means thereto, and hence this science requires such a deduction for its own sake. The third problem, now before us, concerns, as it were, the essence and speciality of metaphysics, namely, the occupation of Reason with itself alone, inasmuch as it broods over its own conceptions and the knowledge of objects supposed to arise immediately from them, without having need of the mediation of experience, or indeed without the possibility of being able to attain thereto by its means. Without a satisfactory solution of this problem, Reason can never be just to itself. The empirical use to which Reason limits the understanding, does not exhaust its own function. Each special experience is but a portion of the whole sphere of its domain. But the absolute totality of all possible experience, though in itself no experience, constitutes nevertheless for Reason a necessary problem, to the mere presentation of which it demands quite different conceptions from the pure conceptions of the understanding, the use of which is only immanent, i.e., referable to experience, so far as it can be given; whereas the conceptions of Reason extend to the completeness, i.e., the collective unity of all possible experience, thereby passing beyond any given experience and becoming transcendent.

    As, then, the understanding required the Categories for experience, so Reason contains in itself the ground of Ideas, by which I understand necessary conceptions the subject of which cannot be given in any experience. The latter are as inherent in the nature of Reason as the former in the nature of the Understanding, and if they carry with them an illusion that may easily mislead, this illusion is unavoidable, although we may very well guard ourselves from being misled by it.

    As all illusion consists in the subjective ground of judgment being taken for objective, the self-knowledge of pure Reason, in its transcendent (exaggerated) use, is the only preservative against the aberrations into which Reason falls when it misapplies its function, and refers its transcendent character, concerning only its own subject and its direction in all immanent uses, to the object itself.

    Section Forty-one: The Third Part, Continued

    The distinction between the ideas, or pure conceptions of Reason, and the categories or pure conceptions of the understanding as being cognitions of quite another order, origin, and use, is so important a point in the foundation of a science, destined to contain the system of all these cognitions a priori, that without a division of this kind metaphysics would be simply impossible, or at best an incoherent, clumsy attempt at building a house of cards, without a knowledge of the materials handled, and of their capacity for this or that purpose. If the Critique of Pure Reason had only accomplished the direction of attention to the distinction for the first time, it would have thereby contributed more to the explanation of our conceptions and to the guidance of investigation in the field of metaphysics, than all the fruitless endeavours at solving the transcendental problems of pure Reason that have ever been undertaken, in which the suspicion has never occurred that the field was quite other than that of the pure understanding, and where consequently the conceptions of the understanding and Reason have been classed together as though they were of the same kind.

    Section Forty-two: The Third Part, Continued

    All pure cognitions of the understanding have the peculiarity that their conceptions are given in experience, and their axioms can be confirmed by experience; whereas the transcendent cognitions of Reason are neither given as concerns their ideas in experience, nor can their axioms be confirmed or refuted by experience. Hence the error possibly arising can be detected by nothing else but pure Reason itself, and this is very difficult, because Reason by means of its ideas is naturally dialectic, and this unavoidable illusion can be held in check by no objective and dogmatic investigations of the matter, but solely by the subjectivity of Reason itself as a source of ideas.

    Section Forty-three: The Third Part, Continued

    It has always been my greatest aim in the Critique, not alone to distinguish carefully the modes of cognition, but also to derive from their common source all the conceptions pertaining to them severally, so that I should not only be informed whence they come and hence be able to determine their use with certainty, but also that I should have the altogether unexpected, but priceless, advantage of knowing the numeration, classification, and specification of the conceptions a priori, and, therefore, according to principles. Without this, everything in metaphysics is mere rhapsody, in which one never knows whether what one possesses is sufficient, or whether there may not be something wanting in it; and if so, where. We can certainly only have this advantage in pure philosophy, but of this latter it constitutes the essence.

    As I had found the origin of the categories in the four logical functions of all judgments of the understanding, it was only natural to seek the origin of the ideas in the three functions of the conclusions of Reason. For if such pure conceptions of Reason (transcendental ideas) be once given, they could not, unless they were regarded as innate, be found elsewhere than in the same act of Reason, which, as far as form is concerned, constitutes the logical element of the conclusions of Reason, but so far as it presents the judgments of the understanding as determined with respect, either to one or the other form a priori, [constitutes] the transcendental conceptions of pure Reason.

    The formal distinction of the conclusions of Reason, renders their division into categorical, hypothetical and disjunctive, necessary. The conceptions of Reason based thereon, contain, firstly, the idea of the complete subject (substantial); secondly, the idea of the complete series of conditions; thirdly, the determination of all conceptions in the idea of a complete content of the possible. The first idea is psychological, the second cosmological, and the third theological; and as all three give occasion to a dialectic, each of its own kind, the division of the whole dialectic of pure Reason founded thereupon, is into the Paralogism, the Antinomy, and finally the Ideal of the same. By this division we are fully assured that all demands of pure Reason are here presented, in their completeness; that no single one can fail, because the capacity of Reason itself, as that from which they all take their origin, is thereby completely surveyed.

    Section Forty-four: The Third Part, Continued

    In this general consideration it is noteworthy, that the ideas of Reason, unlike the categories, are not of any service whatever in the use of the understanding in experience, but can be wholly dispensed with in this connection; indeed, they are impediments to the maxims of the understanding’s knowledge of nature, notwithstanding their necessity for another purpose, yet to be determined. Whether the soul be, or be not, a simple substance, can be quite indifferent to us, so far as the explanation of its phenomena is concerned, for we cannot render the conception of a simple essence comprehensible, sensuously or in concreto, by any possible experience; and hence it is quite barren as to the hoped-for insight into the cause of the phenomena; and cannot serve as any principle of explanation for what is afforded, either by internal or external experience. Just as little can the cosmological ideas of the beginning of the world or of the eternity of the world (a parte ante) avail us to explain an occurrence in the world itself. Finally, we must, in accordance with a just maxim of the philosophy of Nature, refrain from all explanation of the order of Nature, which is derived from the will of a Supreme Being, because this is no longer a philosophy of Nature, but a confession that we have finished with the latter. Hence these ideas have quite a different determination of their use from the categories, by means of which, and of the principles based upon them, experience itself is first possible. But our laborious analytic of the understanding would be quite superfluous, if our aim were nothing else but mere knowledge of Nature, such as can be given in experience; for Reason accomplishes its work both in mathematics and natural science, certainly and well, without any of this subtle deduction. Thus our Critique of the understanding combines with the ideas of pure Reason, in an aim placed beyond the empirical use of the understanding, of which we have above said that, in this respect, it is quite impossible, and destitute alike of object and meaning. But there must, nevertheless, be an agreement between that which belongs to the nature of Reason and of the understanding, and the former must contribute to the completion of the latter, and cannot possibly confuse it.

    The solution of this problem is as follows: pure Reason has no particular objects denoted by its ideas which lie outside the field of experience in view, but merely requires completeness of the use of the understanding within the system of experience. This completeness, however, can only be a completeness of principles, but not of intuitions and objects. But in order to represent the former definitely, it regards them as the cognition of an object, a cognition completely determined as regards these rules, but the object of which is only an idea, designed to bring the cognition of the understanding as near as possible to the completeness indicated by that idea.


    1. What is the difference between an Idea of Reason and a Category of the Understanding? How are they similar?

    Section Forty-five: Preliminary Observation on the Dialectic of pure Reason

    We have above (Sections Thirty-three and Thirty-four) shown, that the purity of the categories, from all admixture of sensuous determinations, may mislead Reason into extending its use entirely beyond the range of all experience, to things in themselves; for although they can find no intuition that could lend them meaning and sense in concreto, yet as mere logical functions they may represent a thing in general, notwithstanding that, independently, they are unable to give a definite conception of anything whatever. Such hyperbolical objects are what are termed noumena, or pure essences of the Understanding (better essences of thought), as, for instance, substance, when considered as without permanence in time, or a cause, which does not operate in time, &c., inasmuch as predicates are then attached to them, which serve merely to make the conformability of experience to law possible, and at the same time all the conditions of intuition—under which experience is alone possible—are taken away from them, whereby these conceptions lose all significance. There is, however, no danger of the understanding of itself, unimpressed by laws foreign to it, branching out so rashly into the field of mere essences of thought. But when Reason, which cannot be completely satisfied with an empirical use of the rules of the understanding, requires the completion of this chain of conditions, the understanding is driven out of its own sphere, partly to present objects of experience in a series extended so far that no experience can grasp it, and partly (in order to complete this series) to search for noumena, wholly outside the same, to which it may attach the above chain, and thereby, being at last independent of experience, render its attitude once for all complete. These are the transcendental ideas, which, in accordance with the true but hidden ends of the natural determination of our Reason, are designed not for extravagant conceptions, but merely for the unlimited extension of empirical use; but which, however, by an unavoidable illusion seduce the understanding into a transcendent use, that although deceitful, cannot be kept within the bounds of experience by any resolution, but can only be restrained within [due] limits with pains, and by means of scientific instruction.

    Section Forty-six: “Psychological Idea”

    It has long been observed that the subject proper, in all substances, namely, that which remains over after all accidents (as predicates) have been abstracted, that is, the substantial itself, is unknown, and oft-repeated complaints have been made of these limitations of our insight. But it is to be observed as regards this, that the human understanding is not to be taken to task for not knowing the substantial of things, that is, for not being able to determine it by itself, but rather for expecting to know it definitely, like a given object, when it is a mere idea. Pure Reason requires of every predicate of a thing the subject belonging to it, but to this, which is again necessarily only predicate, it requires a further subject, and so on ad infinitum (or as far as we can reach). But it follows from the above, that nothing to which we can attain is to be taken for an ultimate subject, and that the substantial itself can never be thought by our understanding, however deeply penetrating it may be, not even if the whole of Nature were unveiled before it; because the specific nature of our understanding consists in that it thinks all things discursively, i.e., through conceptions, and hence solely by means of predicates, to which the absolute subject must always be wanting. For this reason all real qualities whereby we cognise bodies, even to impenetrability, which must always present itself as the effect of a force, are simply accidents, the subject of which eludes us.

    Now it seems as though in our own consciousness (the thinking subject) we have this substantial, and indeed in an immediate intuition; for all predicates of the internal sense refer to the ego, the subject, and this cannot be thought of as predicate of any other subject whatever. Here, then, the completeness in the connection of the given conceptions as predicates of a subject, not merely an idea, but an existence, namely, the absolute subject itself, seem to be given in experience. But this experience is vain, for the ego is no conception at all, but merely a designation of the object of the internal sense, so far as we can cognise it by no further predicate, and hence in itself it can indeed be no predicate of another thing, and just as little a definite conception of an absolute subject, but only, as in all other cases, the reference of the internal phenomena to their unknown subject. At the same time, this idea (which serves well enough, as regulative principle, completely to annihilate all materialistic explanations of the internal phenomena of our soul) occasions, owing to a perfectly natural misunderstanding, a very plausible argument, by inferring from this supposed cognition of the substantial in our thinking entity, its nature, in so far as the knowledge of the same falls entirely outside the content of experience.

    Section Forty-seven: “Psychological Idea”, Continued

    This thinking self (the soul) may however, as the ultimate subject of thought, which cannot be conceived as the predicate of another thing, be called substance; but this conception remains wholly barren, and void of all results, if permanence, which makes the conception of substances in experience fruitful, cannot be proved of it.

    But permanence can never be proved from the conception of a substance, as a thing in itself, but only for the purposes of experience. The above has been fully explained in the first analogy of experience (in the CPR), and, if this demonstration be not accepted, the attempt need only be made as to whether it is possible to prove, from the conception of a subject, not existing as the predicate of some other thing, that its existence is thoroughly permanent, and that neither in itself, nor through any natural cause, can it arise or pass away. Such synthetic propositions a priori can never be proved in themselves, but only with reference to things as objects of possible experience.

    Section Forty-eight: “Psychological Idea”, Continued

    When from the conception of the soul as substance we infer its permanence, this can be only valid of it as an object of possible experience, and not as a thing in itself, outside all possible experience. Now the subjective condition of all our possible experience is life; consequently, the permanence of the soul can only be inferred in life, for the death of man is the end of all experience, of which the soul is an object, unless the contrary be proved, and this is precisely the question. Hence, the permanence of the soul can only be proved in the life of man (the proof of which will not be required of us), but not after death, which is the real point at issue, for the general reason that the conception of substance, viewed as necessarily conjoined with the conception of permanence, is only [based on] an axiom of possible experience, and therefore only serviceable for the purposes of the latter.

    Section Forty-nine: “Psychological Idea”, Continued

    That something real not merely corresponds but must correspond to our external perceptions, can be proved as concerns experience, but not as a connection of things in themselves. This is as much as to say, that something of an empirical kind, as phenomenon in space, exists outside us, can be proved; for with objects, other than those belonging to a possible experience, we have nothing to do, because, inasmuch as they can be given in no experience, they are to us nothing. That is empirically outside me which can be intuited in space, and as the latter, together with all the phenomena it contains, belongs to the presentations, whose connection according to the laws of experience proves their objective reality, just as much as the connection of the phenomena of the internal sense proves the reality of my soul, as an object of the internal sense; so, by means of external experience, I am just as conscious of the reality of bodies as external phenomena in space, as I am of the existence of my soul in time by means of the internal experience, which I also cognise only through phenomena, as an object of the internal sense, [that is, as] constituting an internal condition, of which the essence in itself, lying at the foundation of these phenomena, is unknown to me. The Cartesian idealism only distinguishes external experience from dream; its regularity being the criterion of the truth of the one as against the irregularity and false illusion of the other. It presupposes, in both of them, space and time as conditions of the reality of the objects, and only asks whether the objects of our external sense, which when awake we meet with in space, are really to be found therein, and in the same way whether the object of the internal sense, the soul, really exists in time; in other words, whether experience can afford certain criteria for the distinction between truth and imagination. Now this doubt may be easily decided, and we always do decide it in common life, in that we investigate the connection of the phenomena in both according to universal laws of experience, and we cannot doubt, when the presentation of external things thoroughly agrees with these, that they constitute reliable experience. Material idealism may accordingly be refuted very easily, inasmuch as phenomena qua phenomena are only considered as to their connection in experience; and it is just as certain an experience that bodies exist outside ourselves (in space), as that I myself according to the presentation of the internal sense exist (in time); for the conception of outside ourselves, denotes simply existence in space. But as the I in the proposition I am, signifies not merely the object of internal intuition (in time) but the subject of consciousness, so in the same way body signifies not merely the external intuition (in space), but also the thing in itself at the basis of this phenomenon, and hence the question as to whether bodies (as phenomena of the external sense) exist apart from my thoughts as bodies, may, in the nature of things, be denied without hesitation. But there is no difference as to the question, whether I myself as phenomenon of the internal sense (soul, according to the empirical psychology) exist in time, apart from my power of presentation, for this must be just as much denied. In the same way, everything when reduced to its true meaning is decided and certain. Formal idealism (otherwise called transcendental by me) really refutes the material or Cartesian [idealism]. For if space be nothing but a form of my sensibility, it is just as real as a presentation in me as I am myself, and the question only turns on the empirical truth of phenomena in the same. If this, however, be not the case, but space and the phenomena [contained] therein are something existing outside ourselves, all criteria of experience, apart from our perception, can never prove the reality of the objects external to us.


    Kant mounts a different attack on the “Psychological Idea” in the Critique. What follows is an excerpt from the B edition of the “Paralogisms [fallacies] of Pure Reason”.

    The CPR, “The Paralogisms of Pure Reason”

    Now,4 as the proposition “I think” (in the problematical sense) contains the form of every judgement in general and is the constant accompaniment of all the categories, it is manifest that conclusions are drawn from it only by a transcendental employment of the understanding. This use of the understanding excludes all empirical elements; and we cannot, as has been shown above, have any favourable conception beforehand of its procedure. We shall therefore follow with a critical eye this proposition through all the predicaments of pure psychology …

    Before entering on this task, however, the following general remark may help to quicken our attention to this mode of argument. It is not merely through my thinking that I cognize an object, but only through my determining a given intuition in relation to the unity of consciousness in which all thinking consists. It follows that I cognize myself, not through my being conscious of myself as thinking, but only when I am conscious of the intuition of myself as determined in relation to the function of thought. All the modes of self-consciousness in thought are hence not conceptions of objects (conceptions of the understanding—categories); they are mere logical functions, which do not present to thought an object to be cognized, and cannot therefore present my Self as an object. Not the consciousness of the determining, but only that of the determinable self, that is, of my internal intuition (in so far as the manifold contained in it can be connected conformably with the general condition of the unity of apperception in thought), is the object.

    1. [On the inference from ‘I think’ to ’I exist as a substance]—In all judgements I am the determining subject of that relation which constitutes a judgement. But that the I which thinks, must be considered as in thought always a subject, and as a thing which cannot be a predicate to thought, is an apodeictic and identical proposition. But this proposition does not signify that I, as an object, am, for myself, a self-subsistent being or substance. This latter statement—an ambitious one—requires to be supported by data which are not to be discovered in thought; and are perhaps (in so far as I consider the thinking self merely as such) not to be discovered in the thinking self at all.

    1. ‘It is raining outside; therefore, there is an It that rains.’ How is Descartes’s cogito inference analogous?

    1. [On the inference from ‘I think’ to ’I exist as a simple substance]—That the I or ego of apperception, and consequently in all thought, is singular or simple, and cannot be resolved into a plurality of subjects, and therefore indicates a logically simple subject—this is self-evident from the very conception of an ego, and is consequently an analytical proposition. But this is not tantamount to declaring that the thinking ego is a simple substance—for this would be a synthetical proposition. The conception of substance always relates to intuitions, which with me cannot be other than sensuous, and which consequently lie completely out of the sphere of the understanding and its thought: but to this sphere belongs the affirmation that the ego is simple in thought. It would indeed be surprising, if the conception of “substance,” which in other cases requires so much labour to distinguish from the other elements presented by intuition—so much trouble, too, to discover whether it can be simple (as in the case of the parts of matter)—should be presented immediately to me, as if by revelation, in the poorest mental representation of all.
    2. [On the inference from ‘I think’ to ’I am the same thinking substance over time]—The proposition of the identity of my Self amidst all the manifold representations of which I am conscious, is likewise a proposition lying in the conceptions themselves, and is consequently analytical. But this identity of the subject, of which I am conscious in all its representations, does not relate to or concern the intuition of the subject, by which it is given as an object. This proposition cannot therefore enounce the identity of the person, by which is understood the consciousness of the identity of its own substance as a thinking being in all change and variation of circumstances. To prove this, we should require not a mere analysis of the proposition, but synthetical judgements based upon a given intuition.
    3. [On the inference from ‘I think’ to ’I exist as a thinking substance distinct from any material substance]—I distinguish my own existence, as that of a thinking being, from that of other things external to me—among which my body also is reckoned. This is also an analytical proposition, for other things are exactly those which I think as different or distinguished from myself. But whether this consciousness of myself is possible without things external to me; and whether therefore I can exist merely as a thinking being (without being man)—cannot be known or inferred from this proposition.

    Thus we have gained nothing as regards the cognition of myself as object, by the analysis of the consciousness of my Self in thought. The logical exposition of thought in general is mistaken for a metaphysical determination of the object.

    Our Critique would be an investigation utterly superfluous, if there existed a possibility of proving a priori, that all thinking beings are in themselves simple substances, as such, therefore, possess the inseparable attribute of personality, and are conscious of their existence apart from and unconnected with matter. For we should thus have taken a step beyond the world of sense, and have penetrated into the sphere of noumena; and in this case the right could not be denied us of extending our knowledge in this sphere, of establishing ourselves, and, under a favouring star, appropriating to ourselves possessions in it. For the proposition: “Every thinking being, as such, is simple substance,” is an a priori synthetical proposition; because in the first place it goes beyond the conception which is the subject of it, and adds to the mere notion of a thinking being the mode of its existence, and in the second place annexes a predicate (that of simplicity) to the latter conception—a predicate which it could not have discovered in the sphere of experience. It would follow that a priori synthetical propositions are possible and legitimate, not only, as we have maintained, in relation to objects of possible experience, and as principles of the possibility of this experience itself, but are applicable to things in themselves—an inference which makes an end of the whole of this Critique, and obliges us to fall back on the old mode of metaphysical procedure. But indeed the danger is not so great, if we look a little closer into the question.

    There lurks in the procedure of rational Psychology a paralogism, which is represented in the following syllogism:

    1. That which cannot be cogitated otherwise than as subject, does not exist otherwise than as subject, and is therefore substance.
    2. A thinking being, considered merely as such, cannot be cogitated otherwise than as subject. Therefore,
    3. It exists also as such, that is, as substance.

    In the major premise [i] we speak of a being that can be cogitated generally and in every relation, consequently as it may be given in intuition. But in the minor premise [ii] we speak of the same being only in so far as it regards itself as subject, relatively to thought and the unity of consciousness, but not in relation to intuition, by which it is presented as an object to thought. Thus the conclusion is here arrived at by equivocation.

    That this famous argument is a mere paralogism, will be plain to any one who will consider the general remark which precedes our exposition of the principles of the pure understanding, and the section on noumena. For it was there proved that the conception of a thing, which can exist per se—only as a subject and never as a predicate, possesses no objective reality; that is to say, we can never know whether there exists any object to correspond to the conception; consequently, the conception is nothing more than a conception, and from it we derive no proper knowledge. If this conception is to indicate by the term substance, an object that can be given, if it is to become a cognition, we must have at the foundation of the cognition a permanent intuition, as the indispensable condition of its objective reality. For through intuition alone can an object be given. But in internal intuition there is nothing permanent, for the ego is but the consciousness of my thought. If then, we appeal merely to thought, we cannot discover the necessary condition of the application of the conception of substance—that is, of a subject existing per se—to the subject as a thinking being. And thus the conception of the simple nature of substance, which is connected with the objective reality of this conception, is shown to be also invalid, and to be, in fact, nothing more than the logical qualitative unity of self-consciousness in thought; whilst we remain perfectly ignorant whether the subject is composite or not.


    1. What would Kant make of the argument (in Descartes’s Sixth Meditation) for the real distinction between mind and body?

    We now return to the Prolegomena. Having finished with the Idea of the self as substance, Kant moves on to the Idea of the world of experience as a whole.

    The Prolegomena, Section Fifty: “The Cosmological Idea”

    This product of pure Reason in its transcendent use is its most remarkable phenomenon, and is moreover the one most powerful in awakening philosophy out of its dogmatic slumber, and in urging it on, to the heavy tasks of the Critique of Reason.

    I term this idea cosmological, because it always takes its object from the world of sense, and only requires those [conceptions] whose object is an object of sense, being therefore native [immanent] and not transcendent, and consequently, thus far, no idea; while, on the other hand, to conceive the soul as a simple substance, is equivalent to conceiving an object (the simple) which cannot be presented to the senses. But notwithstanding this, the cosmological idea extends the connection of the conditioned with its condition (whether mathematical or dynamical) so far, that experience can never reach it, and hence remains, as regards this point, always an idea, the object of which can never be adequately given in any experience whatever.


    1. In what way does the “Cosmological Idea” remain within the bounds of possible experience? In what way does it go beyond these bounds?

    Section Fifty-one: “The Cosmological Idea”, Continued

    It is here that the usefulness of a system of categories shows itself so plainly and unmistakably, that, even were there not several other proofs of it, this alone would quite sufficiently demonstrate its indispensableness in the system of pure Reason. There are not more than four of these transcendent ideas, as many as there are classes of categories; but each of them is only concerned with the absolute completion of a series of conditions to a given conditioned. In accordance with these cosmological ideas there are four dialectical assertions of pure Reason, which, inasmuch as they are dialectical, show that to each one is opposed a contradictory assumption, on equally plausible principles of pure Reason; and this is a conflict no metaphysical art of the subtlest distinction can avoid, but which compels philosophers to go back to the primary sources of pure Reason. The above antinomy, which is not arbitrarily invented, but has its basis in the nature of human Reason, and is hence unavoidable and never-ending, contains the following four theses together with their antitheses:

    Thesis
    1 The world has a beginning in time and space
    2 Everything in the world consists of simple [parts].
    3 There are in the world causes through freedom.
    4 In the series of world-causes there exists a necessary being.
    Antithesis
    1 The world is infinite in time and space
    2 There is nothing simple, but everything is composite.
    3 There is no freedom, but all is Nature
    4 There is nothing necessary, but in this series all is contingent.

    Section Fifty-two: “The Cosmological Idea”, Continued

    The above is the most remarkable phenomenon of the human Reason, of which no instance can be shown in any other sphere. If, as generally happens, we regard the phenomena of the world of sense as things in themselves; if we assume the principles of their connection as universal of things in themselves and not merely as principles valid of experience, as is usual and indeed unavoidable without our Critique; then an unexpected conflict arises, never to be quelled in the ordinary dogmatic way, because both theses and antitheses can be demonstrated by equally evident, clear and irresistible proofs—for I pledge myself as to the correctness of all these proofs—and Reason thus sees itself at issue with itself, a state over which the sceptic rejoices, but which must plunge the critical philosopher into reflection and disquiet.

    Section Fifty-two (B): “The Cosmological Idea”, Continued

    One may bungle in metaphysics in many ways, without any danger of being detected in fallacy. For if we only do not contradict ourselves, which is quite possible in synthetic propositions, even though they may be purely invented, we can never in such cases (the conceptions we connect, being mere ideas, which as to their whole content can never be given in experience) be refuted by experience. For how should we decide by experience whether the world exists from eternity, or has a beginning? or whether matter is infinitely divisible, or consists of simple parts? Such conceptions cannot be given in any, even the largest possible experience, and therefore the fallacy of the propositions maintained or denied cannot be discovered by that test.

    The only possible case in which Reason could reveal against its will its secret dialectic, fallaciously given out by it as dogmatic, would be, if it grounded an assertion on a universally admitted axiom, and from another, equally conceded, drew a precisely opposite conclusion, with the greatest logical accuracy. This case is here realised, and indeed in respect of four natural ideas of Reason whence four assertions on the one hand, and just as many counter-assertions on the other, arise, each as a correct consequence from universally admitted premises, and thereby reveal the dialectical illusion of pure Reason in the use of these principles, which must otherwise have been for ever hidden.

    Here then is a decisive attempt, which must necessarily disclose to us the fallacy lying hidden in the assumptions of Reason. Of two mutually contradictory propositions, both cannot be false, unless the conception at their basis be itself contradictory. For instance, two propositions, a square circle is round and a square circle is not round, are both false. For as regards the first, it is false that the [figure] mentioned is round, because it is square, but it is also false that it is not round, or that it is square, because it is a circle. For in this consists the logical mark of the impossibility of a conception, that under the same assumption two contradictory propositions would be equally false; in other words, because no middle can be conceived between them, nothing at all is cogitated by that conception.

    Section Fifty-two (C): “The Cosmological Idea”, Continued

    Now, a contradictory conception like the foregoing lies at the basis of the two first antinomies, which I call mathematical, because they are concerned with the addition or division of things similar in Nature; and thence I explain how it happens that thesis and antithesis are alike false.

    When I speak of objects in time and space, I do not speak of things in themselves, because of these I know nothing, but only of things in the phenomenon, in other words, of experience, as the special mode of the cognition of objects, which is alone vouchsafed to man. I must not say that what I think in space or in time exists in itself in space and time apart from this my thought; for I should then contradict myself, because space and time, together with the phenomena in them, are nothing existing in themselves and apart from my presentations, but are themselves only modes of presentations, and it is obviously contradictory to say that a mere mode of our presentation exists outside our presentation. The objects of sense exist then only in experience; and to give them a special substantive existence for themselves, apart from or before the latter, is equivalent to imagining that experience can be present without or before experience.

    Now, when I inquire as to the size of the world in space and time, it is for all my conceptions just as impossible to say, it is infinite, as it is finite. For neither of them can be contained in experience, because experience is neither possible respecting an infinite space, or an infinite time, or the boundary of the world by an empty space or a previous empty time; these [things] are only ideas. Hence as regards either one or the other kind of determinate quantity, it must lie in the world itself, separate from all experience. But this contradicts the conception of a world of sense, which is only a content of experience, whose reality and connection takes place in presentation, namely, in experience, because it is not a thing in itself, but is itself nothing but a mode of presentation. It follows from the above, that, as the conception of a self-existent world is in itself contradictory, the solution of the problem as to its size will be always fallacious, no matter whether it be affirmatively or negatively attempted.

    The same applies to the second antinomy, which concerns the division of phenomena. For these are mere presentations, and the parts exist merely in their presentation, and therefore in their division; in other words, in a possible experience in which they are given, and they only extend as far as the latter reaches. To assume that a phenomenon, for instance, that of body, contains all parts in itself, before all experience, to which nought but possible experience can ever attain, is equal to giving to a mere appearance, which can exist only in experience, a special existence preceding experience, or to say that mere presentations are there before they are met with in the faculty of presentation, which contradicts itself; and so, consequently, does every solution of this misunderstood problem, whether it be maintained that bodies consist of infinitely many parts, or of a finite number of simple parts.

    The CPR: “The Third Antinomy”

    Thesis—Causality according to the laws of nature, is not the only causality operating to originate the phenomena of the world. A causality of freedom is also necessary to account fully for these phenomena.

    Antithesis—There is no such thing as freedom, but everything in the world happens solely according to the laws of nature.

    The Prolegomena, Section Fifty-three: “The Cosmological Idea”, Continued

    In the first class of antinomy (the mathematical), the fallacy of the assumption consisted in that what is self-contradictory (namely, phenomenon and thing in itself) was represented as capable of union in one idea. But as regards the second, or dynamical class of antinomy, the fallacy of the assumption consists in that what is capable of union is represented as contradictory, and consequently, as in the first case, both contradictory assertions were false; so here, where they are opposed to one another merely through misunderstanding, both may be true.

    The mathematical connection necessarily presupposes homogeneity in the connected (in the conception of quantity), while the dynamical by no means requires this. Where the quantity of the extended is concerned, all the parts must be homogeneous, both with each other and with the whole; whereas in the connection of cause and effect, although homogeneity may also be met with, it is not necessary. For the conception of causality, by means of which a thing is posited by something quite distinct therefrom, at least does not require it. If the objects of the sense-world were taken for things in themselves, and the above-cited laws of Nature for laws of things in themselves, the contradiction would be un avoidable. In the same way, if the subject of freedom were presented like other objects as mere appearance, the contradiction would be equally unavoidable; for the same thing would be at once affirmed and denied of the same kind of object in the same sense. But if natural necessity be referred merely to phenomena, and freedom merely to things in themselves, no contradiction arises, in assuming or admitting both kinds of causality, however difficult or impossible it may be to render the latter kind comprehensible.

    In the phenomenon, every effect is an event, or something that happens in time; a determination of the causality of its cause (a state of the same), must precede it, upon which it follows according to a uniform law. But this determination of the cause to causality must also be something that takes place, or happens. The cause must have begun to act, otherwise between it and the effect, no succession in time could be conceived. The effect would always have existed, as well as the causality of the cause. Thus, among phenomena, the determination of the cause to the effect must also have arisen, and therefore be just as much as its effect, an event which, in its turn, must have a cause, and so on; and consequently, necessity must be the condition according to which the efficient causes are determined. If, on the other hand, freedom be a characteristic of certain causes of phenomena, it must, as regards the latter as events, be a faculty of beginning them from itself (sponte), i.e., without the causality of the causes themselves having begun, and hence another ground would be necessary to determine its beginning. In that case, however, the cause, as to its causality, must not be subject to time determinations of its state; that is, it must not be phenomenon, but it must be regarded as a thing in itself, and its effects only, as phenomena. If one can conceive such an influence of the essences of the understanding on phenomena without contradiction, though necessity would attach to all connection of cause and effect in the sense-world, yet of the cause which is itself no phenomenon, although it lies at the foundation of the latter, freedom would be admitted. Thus Nature and Freedom can be attributed without contradiction to the same thing, at one time as phenomenon, at another, as thing in itself.

    We have a faculty within us, not only standing in connection with its subjective determining grounds, which are the natural causes of its actions, and in so far the faculty of a being, belonging to phenomena, but also referable to objective grounds, though these are merely ideas, in so far as they can determine this faculty; and this connection is expressed by ought. The above faculty is termed Reason, and when we contemplate a being (man) simply according to this subjectively determining Reason, it cannot be regarded as an essence of sense, but the quality thought of is the quality of a thing in itself, of the possibility of which, namely, the ought of that which has never happened, and yet the activity of which can be the determination and cause of actions, whose effect is phenomenal in the sense-world, of this we can form no conception whatever. At the same time, the causality of Reason as concerns its effects in the sense-world would be freedom, so far as objective grounds, which are themselves ideas, are regarded as determining these effects. For its action would then depend not on subjective, and therefore on time-conditions, nor on natural laws, serving to determine these, since grounds of Reason in general would furnish the rule for actions according to principles, without the influence of circumstances, time, or place.

    What I adduce here, is merely meant as an instance for the sake of intelligibility, and does not necessarily belong to our question, which must be decided from mere conceptions, independently of the qualities we meet with in the real world.

    I can say now without contradiction, that all actions of rational beings, inasmuch as they are phenomena, met with in any experience, are subject to necessity; but precisely the same actions, with reference to the rational subject, and its capacity of acting according to mere Reason, are free. For what is demanded by necessity? Nothing more than the determinability of every event in the sense-world according to uniform laws; in other words, a reference to Cause in the phenomenon, whereby the thing in itself, lying at its foundation, and its causality, remains unknown. But I say: the natural law subsists alike, whether the rational being [acting] from Reason, and hence through freedom, be the cause of the effects in the sense-world, or whether these are determined by other grounds than those of Reason. For in the first case, the action happens according to maxims, whose effect in the phenomenon will be always in accordance with uniform laws; in the second case, if the action does not happen according to principles of Reason, it is subordinated to the empirical laws of the sensibility, and in both cases the effects are connected according to uniform laws; more than this we do not require to [constitute] natural necessity, nay, more we do not know respecting it. But in the first case, Reason is the cause of these natural laws, and is hence free; in the second case, the effects follow the mere natural laws of the sensibility, because Reason exercises no influence upon them; Reason, however, is not on this account itself determined by the sensibility (which is impossible), and is consequently in this case also free. The freedom does not hinder the natural law of the phenomena, any more than the latter interferes with the freedom of the practical use of Reason, which stands in connection with things in themselves as determining grounds.

    In this way, the practical freedom, namely, that by which Reason has causality, according to objective determining grounds, is saved, without natural necessity being curtailed in the least, in respect of the same effects as phenomena. The above may also be serviceable as an explanation of what we had to say regarding transcendental freedom, and its union with natural necessity (in the same subject, but not taken in the same connection). For as to this, every beginning of the action of a being, from objective causes, so far as its determining grounds are concerned, is always a first beginning, although the same action in the series of phenomena is only a subaltern beginning, necessarily preceded by a state of the cause determining it, and itself determined by a [state] immediately preceding; so that without falling into contradiction with the laws of Nature, we may conceive of a faculty in rational beings, or in beings generally, in so far as their causality is determined in them, as things in themselves, by which a series of states is begun of themselves. For the relation of the action to objective grounds of Reason is no relation in time; here, what determines the causality does not precede the action according to time, because such determining grounds [as these] do not present a reference of the objects to sense, or, in other words, to causes in the phenomenon, but to determining causes, as things in themselves, which are not subordinated to time-conditions. Hence, the action may be viewed with regard to the causality of Reason as a first beginning, but at the same time, as regards the series of the phenomena, as a merely subordinate beginning, and without contradiction, in the former aspect as free, and in the latter, inasmuch as it is merely phenomenon, as subordinate to natural necessity.

    As concerns the fourth antinomy, it is solved in the same manner as is the conflict of Reason with itself, in the third. For if the cause in the phenomenon be only distinguished from the cause of the phenomena, so far as they can be considered as things in themselves, both propositions can subsist beside one another, namely, that no cause takes place anywhere in the sense-world (according to similar laws of causality) whose existence is absolutely necessary; while, on the other hand, this world may be connected with a necessary being as its cause, though of another kind, and according to other laws; the incompatibility of the above two propositions simply resting on the misunderstanding by which what is merely valid of phenomena is extended to things in themselves, both being mixed up in one conception.


    1. In the case of the third and fourth antinomies, the mistake lies in thinking that the propositions are incompatible, when in fact they are. How does Kant make sense of this? How can both propositions in each pair be true?

    Section Fifty-four: “The Cosmological Idea”, Continued

    This is the arrangement and solution of the whole antinomy, in which Reason finds itself involved, in the application of its principles to the sense-world, and of which even this (the mere arrangement) would be itself a considerable service to the knowledge of the human Reason, even though the solution of the conflict should not fully satisfy the reader, who has here a natural illusion to combat, which has only recently been presented to him as such, and which he has previously regarded as true. For one consequence of this is inevitable, namely, that seeing it is quite impossible to get free of this conflict of Reason with itself, so long as the objects of the sense-world are taken for things in themselves, and not for what they are in reality, namely, mere phenomena, the reader is necessitated thereby again to undertake the deduction of all our knowledge a priori, and its examination as given by me, in order to come to a decision in the matter. I do not require more [than this] at present; for if he has but first penetrated deeply enough into the nature of pure Reason, the conceptions by which the solution of this conflict of Reason is alone possible, will be already familiar to him, without which circumstance I cannot expect full credit even from the most attentive reader.


    To think about: in some sense, Kant can be called a ‘compatibilist’: he thinks it’s at least possible that determinism and free will should co-exist. Is this any different from Hume’s compatibilism? If so, how?

    Section Fifty-five: “The Theological Idea”

    The third transcendental idea, which furnishes material to the most important, but, when merely conducted speculatively, to the exaggerated (transcendent) and thereby dialectical use of Reason, is the ideal of pure Reason. Reason does not here, as with the psychological and cosmological ideas, start from experience, and is not, by a [progressive] raising (Steigerung) of the grounds, misled into an endeavour to contemplate the series in absolute completeness, but wholly breaks therewith, and from mere conceptions of what would constitute the absolute completeness of a thing in general, and consequently by means of the idea of a most perfect original being, descends to the determination of the possibility, and thereby also to the reality, of all other things. For this reason, the mere assumption of a being, which although not given in the series of experience, is nevertheless conceived for the sake of experience, to render comprehensible the connection, order, and unity of the latter, that is, the Idea is more easily distinguishable from the conceptions of experience [in the present] than in the foregoing cases. The dialectical illusion therefore arising from our holding the subjective conditions of our thought for the objective conditions of things themselves, and a necessary hypothesis for the satisfaction of our Reason for a dogma, may be easily exposed to view; and hence I have nothing further to recall on the assumptions of the transcendental theology, for what the Critique has said on this point is comprehensible, clear, and decisive.

    Section Fifty-six: General Remark on the Transcendental Ideas.

    The objects given us through experience are in many respects incomprehensible, and there are many problems to which the natural law leads us, when it is carried to a certain height, (though always in accordance with these laws,) which can never be solved; as for instance, how it is that substances attract one another. But, if we entirely leave Nature, or in the progress of its connection overstep all possible experience, and thereby immerse ourselves in mere ideas, we cannot then say that the object is incomprehensible, and that the nature of things places insoluble problems before us; for we have in that case, nothing whatever to do with Nature or with given objects, but merely with conceptions, having their origin simply in our Reason, and with mere essences of thought, in respect of which all problems arising from the conception of the same, can be solved, because Reason can and must certainly give a complete account of its own procedure. As the psychological, cosmological, and theological ideas, are simply conceptions of Reason, not capable of being given in any experience, so the problems which Reason in respect thereof places before us, are not propounded by the objects, but by mere maxims of Reason for its own satisfaction, and must be capable of being adequately answered in their totality, which is effected by showing them to be principles [designed] to bring the use of our understanding to thorough agreement, completeness and synthetic unity, and which are in so far valid merely of experience, but of the whole of the latter.

    Now, although an absolute whole of experience is impossible, the idea of a whole of knowledge according to principles in general, is what alone can procure a particular kind of unity, namely, that of a system, without which our knowledge is nothing but a patchwork, and cannot be used for the highest end (which is always the system of all ends); by this I understand not merely the practical, but also the highest end of the speculative use of Reason.

    The transcendental ideas express, then, the specific destiny of Reason, namely, as being a principle of the systematic unity of the use of the understanding. But when this unity of the mode of cognition be viewed as though it depended upon the object of cognition; when we hold that which is merely regulative for constitutive, and persuade ourselves that we can extend our cognition by means of these ideas, far beyond all possible experience in a transcendent manner, notwithstanding that they merely serve to bring experience as nearly as possible to completeness, i.e., to limit its progress by nothing which cannot belong to experience—then this is a simple misunderstanding in judging the special destiny of our Reason and its principles, and a dialectic, partly confusing the use of Reason in experience, and partly making Reason to be at issue with itself.

    Section Fifty-seven: Conclusion: On the determination of the boundary of pure Reason

    After all the very clear proofs we have above given, it would be absurd for us to expect to cognise more in any object than what belongs to its possible experience, or to lay claim to the least knowledge of anything whatever which would determine its constitution in itself, unless we assume it to be an object of possible experience. For wherewith shall we effect this determination, inasmuch as time, space, and all the conceptions of the understanding, and still more the conceptions derived from empirical intuition or perception in the sense-world would neither have nor could have any other use than merely to make experience possible, and when if we leave out this condition from the pure conceptions of the understanding, they determine no object whatever, and have no significance anywhere.

    But it would be a still greater absurdity for us not to admit things in themselves at all, or to wish to give out our experience for the only possible mode of the cognition of objects, in other words, our intuition in space and time for the only possible intuition, and our discursive understanding for the model of every possible understanding, thereby wishing principles of the possibility of experience to be held for the universal conditions of things in themselves.

    Our principles, which limit the use of Reason to possible experience, might accordingly become transcendent, and the limits of our Reason be given out for the limits of things themselves, … if a careful Critique of the boundaries of our Reason did not keep watch on its empirical use, and set a limit to its pretensions. Scepticism originally arose from metaphysics and its anarchical (Polizeilosen) dialectic. At first, to favour the empirical use of the understanding, it might well give out for nugatory and deceptive all that exceeded this; but gradually, as it became evident that the very same principles which we make use of in experience are a priori, and that they led unobserved, and as it seemed with the same right, still farther than experience reaches, a doubt began to be thrown on the principles of experience themselves. Now as to these there is no danger, for herein a healthy understanding will always assert its rights; but there arose a special confusion in science, which could not determine how far, and why only thus far and no farther, Reason is to be trusted; but this confusion can only be got rid of, and any future relapse prevented, by a formal limitation of the use of our Reason, derived from principles. It is true we cannot form any definite conception of what things in themselves, beyond all possible experience, may be. But we are nevertheless not free to withdraw ourselves wholly from the inquiry as to these; for experience never fully suffices for Reason; it thrusts us ever farther and farther back for the answer to this question, and leaves us as regards its complete solution dissatisfied; as any one can see from the dialectic of pure Reason, which on this account has its valid subjective ground. Who can tolerate [the circumstance] that by the nature of our soul we can attain to the clear consciousness of the subject, and to the conviction that its phenomena cannot be explained materialistically without asking what the soul really is, and if no empirical conception suffices [to explain] this, at least assuming a conception of Reason (of a simple immaterial essence) merely for the above purpose, although we cannot demonstrate its objective reality in any way? Who can satisfy himself in all cosmological questions, as to the size and duration of the world, of freedom or natural necessity, with mere empirical knowledge, since, begin it as we will, every answer given according to the fundamental laws of experience, gives birth to a new question, just as much requiring an answer, and thereby clearly exposing the inadequacy of all physical modes of explanation for the satisfaction of Reason? Finally, who in the face of the thoroughgoing contingency and dependence of all that he can assume and think according to empirical principles, does not see the impossibility of taking his stand on these, and does not feel himself necessarily impelled, in spite of all prohibition against losing himself in transcendent ideas, to seek rest and satisfaction beyond all conceptions he can verify by experience, in that of a Being, of whom the possibility of the idea in itself cannot indeed be apprehended, but which cannot be refuted, because it is a mere being [essence] of the understanding, and without which Reason must remain for ever unsatisfied.

    Boundaries (with extended beings) always presuppose a space, met with, outside a certain definite place, and enclosing it. Limits do not require this, being mere negations affecting a quantity, so far as it has no absolute completeness. Our Reason, however, sees around it as it were a space for the cognition of things in themselves, although it can never have definite conceptions of them, being limited to phenomena.

    As long as the cognition of Reason is homogeneous, no definite boundaries can be conceived therein. In mathematics and natural science the human Reason recognises indeed limits but no boundaries, i.e., [it recognises] that something exists outside itself, to which it can never attain, but not that it can itself be anywhere terminated in its inner progress. The extension of our views in mathematics and the possibility of new inventions reaches to infinity; and the same can be said of the discovery of new qualities in Nature, and of new forces and laws, through continued experience and the union of the same by Reason. But, at the same time, it cannot be mistaken that there are limits here, for mathematics refers only to phenomena, and what cannot be an object of sensuous intuition, such as the conceptions of metaphysics and morals, lies wholly outside its sphere, [in a region] to which it can never lead, and which does not at all require it. There is, then, a continuous progress and approach to these sciences, and as it were a point or line of contact. Natural science will never discover for us the inner [nature] of things, namely, that which is not phenomenon, but which can still serve as the highest ground of the explanation of phenomena. But it does not require this for its physical explanations; nay, if such were offered it from another source (e.g., the influence of immaterial beings), it ought to reject it, and on no account to bring it into the course of its explanations, but invariably to base these on that which pertains to experience as object of sense, and which can be brought into connection with our real perceptions, and empirical laws.

    But metaphysics leads us to boundaries in the dialectical attempts of pure Reason (which are not commenced arbitrarily or rashly, but to which the nature of Reason itself urges us), and the transcendental ideas, as we cannot have intercourse with them, and as they will never allow themselves to be realised, serve, not only to show us the actual boundaries of the use of pure Reason, but also the way to determine them. And this is also the end and use of this natural disposition of our Reason, which has given birth to metaphysics as its pet child, whose generation, like that of everything else in the world, is not to be ascribed to chance, but to an original germ, wisely organised for great ends. For metaphysics is, perhaps more than any other science, rooted in us in its fundamental features by Nature herself, and can by no means be regarded as the product of a voluntary choice or as chance extension in the progress of experiences (from which it is wholly divided).

    Reason, though all its conceptions and laws of the understanding are adequate in the sense-world, does not find any satisfaction for itself in them, for it is deprived of all hope of a complete solution by questions recurring ad infinitum. The transcendental ideas which have this completion for an object are such problems of Reason. It sees clearly that the sense-world cannot contain the completeness [required], and therefore just as little can those conceptions which serve simply to the understanding of the same, namely, space and time, and all that we have adduced under the name of pure conceptions of the understanding. The sense-world is nothing but a chain of phenomena, connected according to universal laws, and has therefore no subsistence for itself, being not properly the thing in itself, and only being necessarily referable to that which contains the ground of this phenomenon, to essences that cannot be cognised merely as phenomena but as things in themselves. Only in the cognition of these can Reason hope to see its desire for completeness in the progress from the conditioned to its conditions, once for all satisfied.

    We have above (Sections Thirty-three and Thirty-four) assigned the limits of Reason in respect of all cognition of mere beings of thought. Now, as the transcendental ideas make the progress up to these necessary, and have thus led us, as it were, to the contact of the full space (of experience) with the void of which we know nothing (to the noumena), we can determine the boundaries of pure Reason. For in all boundaries there is something positive (for instance surface is the boundary of corporeal space and yet is itself a space; line, a space which is the boundary of the surface; point, the boundary of the line, but still [occupying] a position in space), while, on the other hand, limits contain mere negations. The limits assigned in the paragraph cited, are not sufficient, after we have found that something lies beyond them (although we can never know what this may be in itself). For the question is now, what is the attitude of our Reason in this connection of that which we know, with that which we do not know, and never can know? Here is a real connection of the known with a wholly unknown (and something that will always remain unknown), and even if in this the unknown should not become in the least [degree] more known—which is indeed not to be expected—the conception of this connection must be able, notwithstanding, to be determined and reduced to distinctness.

    We are obliged, then, to think of an immaterial essence, an intelligible world, and a highest of all beings (mere noumena), because only in these, as things in themselves, does Reason meet with the completeness and satisfaction it can never hope for from the derivation of phenomena from their homogeneous ground, because they really refer to something distinct from the latter (and therefore wholly heterogeneous), inasmuch as phenomena always presuppose a thing in itself, and indicate this, [it matters not] whether we may know it more closely or not.


    1. If Ideas of Reason cannot be used constitutively—that is, they cannot be taken as showing us the nature of things as they are in themselves—what good are they? Pick one of the Ideas (God, the World, or the Soul), and try to explain how it might be worthwhile, before looking below to see what Kant himself says.

    But as we can never know these beings of the understanding as to what they may be in themselves, that is, determinately, but are obliged nevertheless to assume such in relation to the sense-world, and to connect them with it through Reason, we shall be at least able to cogitate this connection by means of such conceptions as express its relation to the sense-world. For if we cogitate the essence of the understanding, through nothing but pure conceptions of the understanding, we really cogitate thereby nothing definite, and our conception is consequently without meaning; if we cogitate it through qualities borrowed from the sense-world, then it is no longer an essence of the understanding, but is conceived as one of the phenomena, and belongs to the sense-world. We will take an instance from the conception of the Supreme Being.

    The deistic conception is an entirely pure conception of Reason, which, however, only represents a thing containing all reality, without our being able to determine a single one of its [qualities], because for this an instance would have to be borrowed from the sense-world, in which case I should always have to do with an object of sense, and not with something completely heterogeneous, and which cannot be an object of sense. For instance, I attribute understanding to it; but I have no conception whatever of any understanding but of one like my own, namely, of one to which intuitions must be given through the senses, and which occupies itself with reducing these under rules of the unity of the consciousness. But then the elements of my conception would always lie in the phenomenon; yet I was necessitated by the inadequacy of the phenomena to pass beyond this, to the conception of a being in no way dependent on phenomena, or bound up with them, as conditions of its determination. If, however, I sever the understanding from the sensibility in order to have a pure understanding, nothing remains over but the mere form of thought without intuition, by means of which I can cognise nothing determinate as object. For this purpose I should have to conceive another understanding which intuited objects, but of which I have not the least conception, because the human understanding is discursive and can only cognise through universal conceptions. But I am also involved in contradiction if I attribute will to the Supreme Being. For I have this conception only in so far as I derive it from my inner experience, and thereby from the dependence of my satisfaction from objects whose existence we require; but at the foundation of this lies sensibility, which wholly contradicts the pure conception of the Supreme Being. The objections of Hume to Deism are weak, touching no more than the proofs, and never the proposition of the deistic assertion itself. But as regards Theism, which must be arrived at by a closer determination of our, there [viz., in Deism], merely transcendent conception of the Supreme Being, they are very strong, and, according as the conception is constructed, in certain (indeed in all ordinary) cases are irrefragable. Hume always insists, that through the mere conception of an original being, to whom we can attribute none but ontological predicates (eternity, omnipresence, omnipotence) we really think nothing definite, but that qualities expressing an object in concreto must be superadded. It is not enough to say it is Cause, but [we must also say] what is the nature of its causality, as, whether [it operates] through understanding and will; and at this point his attacks on the thing itself, namely, on Theism, commence, whereas before he had only stormed the grounds of proof of Deism, which does not carry any especial danger with it. His dangerous arguments refer entirely to anthropomorphism, which he holds to be inseparable from Theism, and to make it contradictory in itself; while if this be left out, [Theism itself] would also fall, and nothing would remain but a Deism wherewith nothing could be done, which could not avail us for anything, and could not serve as a foundation for religion and morals. If this inevitability of anthropomorphism were certain, the proofs of the existence of a Supreme Being might be what one liked, and all conceded, yet the conception of this Being would never be able to be determined by us, without involving ourselves in contradictions.

    But if with the injunction to avoid all transcendent judgments of pure Reason, we connect the apparently contradictory injunction to proceed to conceptions lying outside the field of its immanent (empirical) use, we shall be aware that both may subsist together, but only on the exact boundary of all admissible use of Reason; for this belongs as much to the field of experience as to that of essences of thought, and we shall be taught thereby, at the same time, how the above remarkable ideas serve simply, for the determination of the boundaries of the human Reason; namely, on the one hand not to extend cognition of experience in an unbounded manner, so that nothing but mere world remains for us to cognise, and on the other hand not to pass beyond the boundaries of experience, or to seek to judge of things outside the latter as things in themselves.

    But we keep to this boundary when we limit our judgment to the relation the world may have to a Being, whose conception itself lies outside all the cognition of which we are capable within the world. For in this case, we do not attribute to the Supreme Being any of the qualities in themselves by which we cogitate objects of experience, and thereby avoid the dogmatic anthropomorphism; but we apply the relations of the same to the world, and thereby allow ourselves a symbolical anthropomorphism, which as a matter of fact only concerns the language and not the object.

    When I say we are obliged to regard the world as though it were the work of a supreme understanding and will, I do not really say more than—as a watch, a ship, a regiment is related to the artisan, shipbuilder or general, so is the sense-world (or all that which constitutes the foundation of this sum-total of phenomena) [related] to the unknown, that I cognise, not indeed according to what it is in itself, but according to what it is for me, namely, in respect of the world, of which I am a part. …

    Section Fifty-nine: Conclusion, Continued

    [O]ur original proposition remains, which is the result of the whole Critique: “that our Reason can never teach more by its principles a priori than simply objects of possible experience, and even of these no more than what can be cognised in experience.” But this limitation does not prevent it from leading us to the objective boundary of experience, namely, the reference to something which is not itself object of experience, but is nevertheless the highest ground of all experience, without however teaching us anything respecting this in itself, but only with reference to its [viz., Reason’s] own complete use as directed to its highest end, within the field of possible experience. But this is also all the use that can be reasonably expected or even wished, as concerns it, and with this we have cause to be content.

    Section Sixty: Conclusion, Continued

    Thus we have fully exhibited metaphysics according to its subjective necessity, as it is really given in the natural disposition of the human Reason, and indeed in what constitutes its essential purpose. We have found in the course of this investigation, that such a merely natural use of such a disposition of our Reason involves us in extravagant dialectical conclusions, partly apparently, and partly really, conflicting [with one another], if no discipline bridles it and keeps it within limits, which is only possible by means of scientific criticism. And, in addition, [we have found] this fallacious metaphysics to be dispensable to the promotion of the knowledge of Nature, and even prejudicial to it. It always remains, notwithstanding, a task worthy of research, to find out the natural ends aimed at by this disposition in our Reason to transcendent conceptions, since everything in Nature must have been originally designed for some useful purpose.

    Such an investigation is here out of place; I confess, moreover, that all I here say respecting the primary ends of Nature is only conjecture, but which may be permitted me in this case, as the question does not concern the objective validity of metaphysical judgments, but refers merely to the natural disposition to the latter, and thus lies outside the system of metaphysics, in that of anthropology.

    When I compare all transcendental ideas whose content constitutes the special problem of the natural, pure Reason, compelling it to leave the mere contemplation of Nature and to pass beyond all possible experience, and in this endeavour to produce the thing (be it knowledge or nonsense) called metaphysics, I believe myself to have discovered that this natural disposition is intended to free our conceptions from the chains of experience and the limits of the mere contemplation of Nature, in so far that it may at least see a field opened before it, containing mere objects for pure Reason, which cannot be arrived at by any sensibility. The purpose is not, indeed, to occupy ourselves speculatively with these objects, (because we can find no firm ground for our feet), but because practical principles, without finding such a space before them for their necessary expectation and hope, could not expand themselves to the universality, Reason indispensably requires, from a moral point of view.

    Now, I find that the psychological idea, however little may be the insight I obtain by its means into the pure nature of the human soul, which is raised above all conceptions of experience, at least sufficiently shows me the inadequacy of the latter, and thereby preserves me from materialism as being a psychological conception of no avail for the explanation of Nature, and besides, as narrowing Reason in its practical aspect. In the same way the cosmological ideas, by the obvious inadequacy of all possible knowledge of Nature to satisfy Reason in its justifiable inquiries, serve to keep us from the Naturalism which proclaims Nature for self-sufficing. Finally, as all natural necessity in the sense-world is invariably conditioned, inasmuch as it always presupposes dependence of things on one another, and, as unconditioned necessity must be sought for in the unity of a Cause separate from the sense-world, (but the causality of which, if it were mere Nature, could yet never render comprehensible the existence of the contingent as its consequence;) [this being so,] Reason frees itself by means of the theological idea from fatalism, as well from that of a blind natural necessity in the coherence of Nature, without a first principle, as in the causality of this principle itself, and leads to the conception of a cause through freedom, in other words, a supreme intelligence. Thus the transcendental ideas serve, if not to instruct us positively, at least to do away with the audacious assertions of materialism, naturalism, and fatalism, which narrow the field of Reason, and thereby to procure a place for moral ideas outside the region of speculation; and this, as it seems to me, will in some measure explain the above natural disposition.

    The practical utility a merely speculative science may have, lies outside the boundaries of this science, and hence can be merely viewed as a scholium, and, like all scholia, not as forming a part of the science itself. At the same time, this reference lies at least within the boundaries of philosophy, especially of that which draws from the sources of pure Reason, where the speculative use of Reason in metaphysics must have a necessary unity with its practical use in morals. Hence the unavoidable dialectic of pure Reason in metaphysics must be considered as natural disposition—not merely as an illusion requiring to be resolved, but as a natural institution, as concerns its end—deserving, if possible, to be explained, although this task, being supererogatory, cannot in justice be claimed of metaphysics proper. …

    And thus I conclude the analytical solution of the problem I had myself proposed—How is metaphysics at all possible? having proceeded from that in which its use is really given, at least in its consequences, to the grounds of its possibility.


    1. Kant insists on the thinkability—though not on the actuality—of God, of the soul, and of causality through freedom. Is he entitled to this? Think back to the limitations Kant imposed on the use of the Categories earlier in the Prolegomena.

    Solution of the General Problem of the Prolegomena: How is Metaphysics Possible as Science?

    Metaphysics, as a natural disposition of Reason, is real, but it is also, in itself, dialectical and deceptive (as was proved in the analytical solution of the third main problem). Hence to attempt to draw our principles from it, and in their employment to follow this natural but none the less fallacious illusion, can never produce science, but only an empty dialectical art, in which one school may indeed outdo the other, but none can ever attain a justifiable and lasting success. In order that, as science, it may lay claim not merely to deceptive persuasion, but to insight and conviction, a Critique of Reason must exhibit in a complete system the whole stock of conceptions a priori, arranged according to their different sources—the Sensibility, the Understanding, and Reason; it must present a complete table of these conceptions, together with their analysis and all that can be deduced from them, but more especially the possibility of synthetic knowledge a priori by means of their deduction, the principles of its use, and finally, its boundaries. Thus criticism contains, and it alone contains, the whole plan well tested and approved, indeed all the means whereby metaphysics may be perfected as a science; by other ways and means this is impossible. The question now is not, however, how this business is possible, but only how we are to set about it; how good heads are to be turned from their previous mistaken and fruitless path to a non-deceptive treatment, and how such a combination may be best directed towards the common end.

    This much is certain: he who has once tried criticism will be sickened for ever of all the dogmatic trash he was compelled to content himself with before, because his Reason, requiring something, could find nothing better for its occupation. Criticism stands to the ordinary school-metaphysics exactly in the same relation as chemistry to alchemy, or as astronomy to fortune-telling astrology. I guarantee that no one who has comprehended and thought out the conclusions of criticism, even in these Prolegomena, will ever return to the old sophistical pseudo-science. He will rather look forward with a kind of pleasure to a metaphysics, certainly now within his power, which requires no more preparatory discoveries, and which alone can procure for Reason permanent satisfaction. For this is an advantage upon which metaphysics alone can reckon with confidence, among all possible sciences; namely, that it can be brought to completion and to a durable position, as it cannot change any further, nor is it susceptible of any increase through new discoveries. Since Reason does not here find the sources of its knowledge in objects and in their intuition (which cannot teach it anything), but in itself; so that when the principles of its possibility are presented completely, and without any misunderstanding, nothing remains for pure Reason to know a priori, or even with justice to ask. The certain prospect of so definite and perfect a knowledge has a special attraction about it, even if all its uses (of which I shall hereafter speak) be set aside.

    All false art, all empty wisdom, lasts its time; but it destroys itself in the end, and its highest cultivation is at the same time the moment of its decline. That as regards metaphysics this time has now come, is proved by the state to which it has declined among all cultivated nations, notwithstanding the zeal with which every other kind of science is being worked out. The old arrangement of the university studies preserves its outlines still, a single academy of sciences bestirs itself now and then, by holding out prizes to induce another attempt to be made therein; but it is no longer counted among fundamental sciences, and any one may judge for himself how an intellectually-gifted man, to whom the term great metaphysician were applied, would take this well-meant, but scarcely by any one, coveted, compliment.

    But although the period of the decline of all dogmatic metaphysics is undoubtedly come, there are many things wanting to enable us to say that the time of its re-birth by means of a thorough and complete Critique of Reason, has already appeared. All transitional phases from one tendency to its opposite pass through the state of indifference, and this moment is the most dangerous for an author, but, as it seems to me, the most favourable for the science. For when, through the complete dissolution of previous combinations, party spirit is extinguished, men’s minds are in the best mood for listening gradually to proposals for a combination on another plan. If I say that I hope that these Prolegomena will perhaps make research in the field of criticism more active, and will offer to the general spirit of philosophy, which seems to be wanting in nourishment on its speculative side, a new and very promising field for its occupation, I can already foresee that every one who has trodden unwillingly and with vexation the thorny way I have led him in the Critique, will ask me on what I ground this hope. I answer—on the irresistible law of necessity.

    That the spirit of man will ever wholly give up metaphysical investigations is just as little to be expected, as that in order not always to be breathing bad air we should stop breathing altogether. Metaphysics will always exist in the world then, and what is more, [exist] with every one, but more especially with reflecting men, who in default of a public standard will each fashion it in his own way. Now, what has hitherto been termed metaphysics, can satisfy no acute mind; but to renounce it entirely is impossible; hence a Critique of pure Reason itself must be at last attempted, and when obtained must be investigated and subjected to a universal test, because otherwise there are no means of relieving this pressing requirement, which means something more than mere thirst for knowledge. …

    Should any one feel himself offended by what is here said, he can very easily refute the accusation if he will only adduce a single synthetic proposition belonging to metaphysics which admits of being demonstrated in a dogmatic manner a priori; for only when he has achieved this shall I allow that he has really advanced the science, even though the proposition in question may be sufficiently confirmed by common experience. No demand can be more moderate, and more fair, and in the event (unquestionably certain) of non-accomplishment, no statement can be juster than that metaphysics as science has not hitherto existed at all.

    I must only forbid two things, in case the challenge be accepted: first, the apparatus of probability and conjecture, which just as ill becomes metaphysics as geometry; and secondly, a decision by means of the magic wand of so-called sound common sense, which every one does not wave, but which regulates itself according to personal characteristics. For as regards the first, nothing can be more absurd than in a system of metaphysics, a philosophy of pure Reason, to attempt to base judgments on probability and conjecture. All that can be known a priori is thereby given out as apodictically certain, and must be proved as such. A geometry or arithmetic might just as well be attempted to be founded on conjectures; (for as concerns the calculus probabilium of the latter, it does not contain probable but perfectly certain judgments, on the degree of possibility in certain cases, under given similar conditions, which in the sum of all possible cases must infallibly follow in accordance with the rule—although in respect of any single instance this is not sufficiently determined). Even in empirical natural science conjectures (by means of induction and analogy) can only be permitted, in such a manner that at least the possibility of what I assume must be quite certain.

    With the appeal to sound common sense we are still worse off, if possible, when we have to do with conceptions and principles, not so far as they are valid in respect of experience, but when they would be given out as valid outside the conditions of experience. For what is sound sense? It is the common understanding rightly used. And what is the common understanding? It is the faculty of the cognition and employment of rules in concreto in contradistinction to the speculative understanding, which is a faculty for the cognition of rules in abstracto. Thus, the common understanding will hardly comprehend the rule that all which happens is determined by means of its cause, and never be able to view this rule in its universal bearing. Hence it requires an example from experience, and when it hears that it points to nothing else but what it had always thought, when a window-pane was broken or a household utensil lost, it understands the axiom and admits it. Common understanding has no farther use, then, than to be able to see its rules confirmed in experience (although they really pertain to it a priori), and therefore to regard them a priori and independently of experience belongs to the speculative understanding, and lies wholly outside the horizon of the common understanding. But metaphysics is exclusively occupied with the latter kind of knowledge, and it is certainly a bad sign of a sound understanding to appeal to a protector, having no right of judgment here, and which one otherwise only looks at askance, except when one sees oneself pressed, and does not know how to advise or help oneself in a speculation.

    A usual resource employed by these false friends of the common human understanding (who sometimes honour it highly, though they generally despise it) is to say: there must be some propositions, immediately certain, and of which one not only requires to give no proof, but no account whatever, as otherwise we should never come to an end of the grounds of our judgments; but in proof of this assertion they can never bring forward anything undoubted, and which they can attribute immediately to the common human understanding (except the axiom of contradiction, which is inadequate to demonstrate the truth of synthetic judgments) and mathematical propositions; as, for instance, that twice two make four, that between two points there is only one straight line, &c. But these are judgments from which those of metaphysics are totally distinct. For in mathematics I can make (construct) all this by my own thinking, representing it to myself as possible through a conception; I gradually add to the one two, the other two, and myself make the number four; or drawing in thought all sorts of lines from one point to another, can only draw one that is similar in all its parts, equal no less than unequal. But I cannot with my whole power of thought bring out from the conception of one thing the conception of something else, the existence of which is necessarily connected with the first, but must call experience to my aid; and although my understanding a priori offers me such a conception, [viz.] causality (though only in reference to possible experience), I cannot present it a priori in intuition, like the conceptions of mathematics, and thus exhibit its possibility a priori, but the conception together with the principles of its use, if it is to be valid a priori (as is required in metaphysics), demands a demonstration and deduction of its possibility, since otherwise we do not know how far it is valid, and whether it can only be used in experience or [may be used] outside [experience]. Hence, in metaphysics as a speculative science of pure Reason, we can never appeal to the common human understanding, but when we are obliged to leave it, and to renounce all pure speculative cognition, which must be always a branch of knowledge, and therefore under certain circumstances metaphysics itself and its teaching, a reasonable faith will be found alone possible, and indeed sufficient to our needs, and perhaps even better for us than knowledge itself. Then the aspect of the matter is quite altered. Metaphysics must be a science, not alone as a whole, but in all its parts, else it is nothing; because in speculation of pure Reason, nothing has a standing but universal notions. But, apart from this, probability and healthy human understanding, have their useful and justifiable employment, but on their own special principles, whose validity always depends on their relation to the practical.

    It is this which I hold myself justified in demanding of a system of metaphysics, as science.


    1. The first Critique was issued in two editions, 1781 and 1787. Standard translations combine the two editions, giving ‘A’ for pages in the first edition, and ‘B’ for those in the second, separated by a ‘/’ when the two editions overlap. The present selection is A 23/B 38–A 25/B 40.
    2. A189/B233–A195/B240
    3. B167–8
    4. B406–B413
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