5.3: David Hume

Of the philosophers discussed here, David Hume (1711-1776) has probably had the greatest influence on contemporary analytic philosophy. The twentieth century begins with a movement known as Logical Positivism that tests the limits of Empiricism. The Empiricism of the Logical Positivists is heavily indebted to Hume.

Hume’s empiricist epistemology is grounded in his philosophy of mind. Hume starts by asking what we have in the mind and where these things come from. He divides our mental representations into two categories, the relatively vivid impressions, these include sensations and feelings, and the less vivid ideas which include memories and ideas produced by the imagination.

What distinguishes impressions from ideas in our experience is just their vividness. The picture of the mind Hume offers is one where all of our beliefs and representations are cooked up out of basic ingredients provided by experience. Our experience gives us only impressions through sense experience and internal impressions like feelings. From this we generate less vivid ideas. Memories are merely faint copies of impressions. Through the imagination we can generate further ideas by recombining elements of ideas we already have. So through impressions we get the idea of a lizard and the idea of a bird. We can then generate the idea of a dragon by imaginatively combining elements of each. In cooking up new ideas from old ideas, the imagination is guided by associating relations like resemblance, contiguity (next-to-ness) and cause and effect. So, for example, an impression of a grapefruit might lead me to think of an orange due to their similarity. The thought of my bicycle might lead me to think of the table saw it is parked next to in the basement. Through the association of cause and effect, my idea of a struck match leads me to the idea of a flame. The last of these principles of association, cause and effect, turns out to be faulty for reasons we will examine shortly.

The imagination is not merely a source of fancy and fiction. The imagination also includes our ability to understand things when we reason well in formulating new ideas from old ones. A priori reasoning, which is reasoning independent of experience, can produce understanding of relations of ideas. Mathematical and logical reasoning is like this. When I recognize the validity of an argument or the logic behind a mathematical proof, the understanding I attain is just a matter of grasping relations between ideas. But a priori reasoning only reveals logical relations between ideas. It tells us nothing about matters of fact. Our ability to understand matters of fact, say truths about the external world, depends entirely on a posteriori reasoning, or reasoning based on experience. As we will see, our ability to reason about matters of fact doesn’t get us very far.

Often our philosophical confusion is the result of having added more than we are entitled to add to our experience when we are striving to understand it. Hume aims to correct many of these errors and, in doing so, he aims to delineate the limits of human knowledge and understanding. As it turns out, we don’t know as much as we commonly suppose, in Hume’s opinion. The result of Hume’s rigorous Empiricism is skepticism about a great many things. Some of Hume’s skeptical results are not so surprising given his Empiricism. Hume is skeptical about objective moral truths, for instance. We don’t get to observe rightness and wrongness in the way we can see colors and shapes, for instance. The idea that there are objective moral truths, according to Hume, is a mistaken projection of our subjective moral sentiments.

Hume is not worried that his subjectivism about morality will lead to moral anarchy. Note that the opinion that it’s OK to do whatever you want is itself a moral opinion. So, for the subjectivist, “anything goes” is no more rationally justified than any other moral opinion. While Hume does think that morality is concerned with subjective sentiments, not objective facts, the lack of objective moral truths won’t corrupt us or undermine the social order because we all have pretty much the same sorts of moral sentiments and we can base a sensible social order on these. While we may feel differently about specific practices or principles, Hume thinks we have a basis for negotiating our moral differences in our more general and more or less universally shared moral sentiments of self-love, love for others, and concern for happiness.

Hume’s skepticism about objective moral truths now strikes many people as common sense. But the empiricist epistemology that leads him to subjectivism about morality also leads him to skepticism about causation, the external world, inductive reasoning, about God, and even about the self. We’ll examine these further skeptical conclusions starting with causation.

Causation

When we examine our everyday idea of causation, Hume says we find four component ideas:

• the idea of a constant conjunction of cause and effect (whenever the cause occurs, the effect follows).
• the idea of the temporal priority of the cause (the cause happens first, then the effect).
• the idea of causes and effects being contiguous (next to each other) in space and time.
• the idea of a necessary connection between the cause and the effect.

So, for instance, the idea that striking a match causes it to light is made up of the idea that whenever similar matches are struck (under the right conditions), they light, plus the idea of the striking happening first, and the idea of the striking and the lighting happen right next to each other in time and space, and, finally, the idea that the striking somehow necessitates or makes the match light. Now let’s consider these component ideas and ask whether they all have an empirical basis in corresponding sense impressions. We do have sense impressions of the first three: the constant conjunction of cause and effect, the temporal priority of the cause, and the contiguity of cause and effect. But Hume argues that we lack any corresponding empirical impression of necessary connections between causes and effects. We don’t observe anything like the cause making the effect occur. As Hume puts the point,

When we look about us towards external objects, and consider the operation of causes, we are never able, in a single instance, to discover any power or necessary connexion; any quality, which binds the effect to the cause, and renders the one an infallible consequence of the other. We only find, that the one does actually, in fact, follow the other. (An Enquiry Concerning Human Understanding, Section VII)

The idea of causes necessitating their effects, according to Hume’s analysis, is a confused projection of the imagination for which we find no basis in experience. For this reason, Hume denies that we have rational grounds for thinking that causes do necessitate their effects.

The External World

All of our reasoning about the external world is based on the idea of causation. So the skepticism that follows from Hume’s skepticism about causation is quite far reaching. Our beliefs about the external world, for instance, are based on the idea that things going on in the external world cause our sense impressions. We have no rational grounds for thinking so, says Hume.

More generally, our evidence for what we can know begins with our impressions, the mental representations of sense experience. We assume that our impressions are a reliable guide to the way things are, but this is an assumption we can’t rationally justify. We have no experience beyond our impressions that could rationally certify that our impressions correspond in any way to an external reality. Our assumption that our impressions do correspond to an external reality is a rationally unsupportable product of our imagination.

Induction

Closely related to Hume’s skepticism about causation is Hume’s skepticism about inductive reasoning. Inductive argument, in its standard form, draws a conclusion about what is generally the case, or what will prove to be the case in some as yet unobserved instance, from some limited number of specific observations. The following is an example of a typical inductive argument:

1. Every observed sample of water heated to well over 100 C has boiled.
2. Therefore, whenever water is heated to well over 100 C, it boils.

Unless every instance of water heated to over 100C in the history of the universe is among the observed instances, we can’t be sure that the conclusion is true given the truth of the premises. It follows that strong inductive arguments like the one above are not deductively valid. But then what justifies the inference from the premise to the conclusion of an inductive argument?

Hume considers the suggestion that every inductive argument has a principle of induction as a suppressed premise, and it is this principle of induction that renders the inference from premises to conclusion rational. This principle of induction tells us roughly that unobserved instances follow the pattern of observed instances. So inductive arguments really go something like this:

1. Every observed sample of water heated to over 100 C has boiled.
2. (Unobserved cases tend to follow the pattern of observed cases)
3. So, whenever water is heated to over 100 C, it boils.

Of course the argument still isn’t valid, but that’s not what we are aiming for in induction. Given the hidden second premise - our principle of induction - we can reasonably hold that the premises taken together give us good grounds to accept that the conclusion is probably true. However, if this principle of induction (2 above) is to render inductive inferences rational, then we need some grounds for thinking that it is true. In considering how this principle of induction is to be justified, Hume presents a dilemma. Since there is no contradiction in denying the principle of induction, it cannot be justified a-priori (independent of our experience as can be done with logical truths). And any empirical argument would be inductive and therefore beg the question by appealing to the very principle of induction that requires support. So, Hume concludes, we have no rational grounds for accepting inductive inferences.

Think about the ramifications of Hume’s skepticism about induction. If inductive argument is not rational, then we have no reason at all to think the sun will rise tomorrow. Here we aren’t worried about improbably possibilities like the sun getting blown to bits by aliens before tomorrow morning. Hume’s argument against the rationality of inductive reasoning implies that all of our experience of the sun regularly rising gives us no reason to think its rising tomorrow is even likely to happen. If this sounds crazy, then we have a problem because it is not easy to find a defect in Hume’s reasoning. This is why philosophers speak of this topic as the Problem of Induction. Very few are prepared to accept Hume’s skepticism about induction. But in the two and a half centuries that have passed since Hume died, we have yet to settle on a satisfactory solution to the problem of induction. We’ll take a closer look at this problem when we take up the Philosophy of Science in the next chapter.

God

Unlike Locke and Berkeley, Hume’s rigorous Empiricism leads him to skepticism about religious matters. To avoid censorship or persecution, critics of religious belief in the 18th century exercised caution in various ways. Hume’s earliest challenge to religious belief, an essay on miracles, was removed from his early work, his Treatise of Human Nature, and published only in his later Enquiries Concerning Human Understanding. In this essay, Hume argues that the belief in miracles can never be rational. A miracle is understood to be a violation of the laws of nature resulting from Divine will. But, argues Hume, the weight of the evidence of our experience overall will always give us stronger reason to mistrust our senses in the case of a seemingly miraculous experience than to doubt the otherwise consistently regular course of events in our experience. Testimony by others of miracles is on even shakier ground.

No testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous than the fact which it endeavors to establish. (Enquiries Concerning Human Understanding, Section 10)

Among educated people in the 18th century, religious belief was thought to be supported not just by Divine revelation, but by our experience of the natural world as well. When we look to the natural world we find impressive harmony in the natural order of things. The various species all seem well suited to their environments and ecological stability is maintained by the various roles organism play in their environments. To the discerning mind in search of an explanation, the order and harmony we find in the world looks very much like the deliberate work of a Divine creator. This line of thought is known as the Argument from Design. Hume’s last work, his posthumously published Dialogues of Natural Religion, aimed to undermine many arguments for the existence of God, including the Design Argument.

According to Hume, the Design Argument is a weak argument by analogy. We have reason to think that machines are the product of human design because we are familiar with their means of production. But we have no analogue in the case of the universe. We have not observed its creation. The alleged similarity of the universe to machines designed by humans is also suspect. We do find regularities in nature, but only in the small corner of nature we are familiar with. The regularity, order, and harmony we do find don’t provide enough of the appearance of design to warrant positing an intelligent designer, according to Hume. But suppose we do think the natural world bears the marks of a designer’s craftsmanship. The only sorts of designers we are familiar with are people like us. But that doesn’t tell us much about what sorts of being could be designers of complex harmonious systems. So even assuming we find the appearance of design in nature, we have little grounds to think that it is the product of a personal god or any sort of entity we can relate to.

Charles Darwin’s theory of evolution by natural selection provides a naturalistic account of the appearance of design in life forms. Thanks to providing a developed naturalistic alternative to the hypothesis of design by a Divine creator, Darwin probably had the greater impact in undermining the design argument for the existence of God. Darwin cites Hume as among his major influences, and there are a number of passages in Hume’s writing that foreshadow insights that Darwin developed.

The Self

Descartes didn’t hesitate to infer the existence of himself from the certainty of his thinking. And it seems obvious to most of us that having thoughts implies the existence of a subject that thinks. Hume is more cautious on this point.

For my part, when I enter most intimately into what I call, I always stumble on some particular perception or other, of heat or cold, light or shade, love or hatred, pain or pleasure. I never can catch myself at any time without a perception, and never can observe anything but the perception.” (Treatise, 1.4.6.3)

The contents of our immediate experience are just particular impressions and ideas. But we have no experience of any single unified self that is the subject of those experiences. The idea of a self, including the idea of the self as a soul, is a fanciful projection from our experiences. All we can say in an empirically grounded way of ourselves, according to Hume, is that we are just a bundle of experiences.

We’ve given just given the briefest sketch of how Hume reaches his assorted skeptical conclusions. There are many further arguments and objections to consider, but hopefully we’ve covered enough to give you an appreciation for how carefully a strict and carefully reasoned Empiricism leads to a variety of skeptical conclusions. Hume’s skepticism about causation and induction may be the most surprising. We often hold up science as the paradigm of human intellectual achievement, and we tend think of science as pretty empirical. Yet Hume’s strict Empiricism seems to undercut science on the key notions of causation and induction. Perhaps scientific inquiry is not as strictly empirical as Hume’s epistemology. Or perhaps, as some have argued, science can get along fine without induction or causation. Still, if we are not comfortable with Hume’s skepticism about causation and induction, this might be cause to reconsider his Empiricism. And perhaps also the skepticism about morality it seems to invite.