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Humanities Libertexts

4.1: Descartes

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  • Rene Descartes (1596 – 1650) lived during an intellectually vibrant time. European scholars had supplemented Catholic doctrine with a tradition of Aristotle scholarship, and early scientists like Galileo and Copernicus had challenged the orthodox views of the Scholastics. Surrounded by conflicting yet seemingly authoritative views on many issues, Descartes wants to find a firm foundation on which certain knowledge can be built and doubts can be put to rest. So he proposes to question any belief he has that could possibly turn out to be false and then to methodically reason from the remaining certain foundation of beliefs with the hope of reconstructing a secure structure of knowledge where the truth of each belief is ultimately guaranteed by careful inferences from his foundation of certain beliefs.

    When faith and dogma dominate the intellectual scene, “How do we know?” is something of a forbidden question. Descartes dared to ask this question while the influence of Catholic faith was still quite strong. He was apparently a sincere Catholic believer, and he thought his reason-based philosophy supported the main tenants of Catholicism. Still he roused the suspicion of religious leaders by granting reason authority in the justification of our beliefs.

    Descartes is considered by many to be the founder of modern philosophy. He was also an important mathematician and he made significant contributions to the science of optics. You might have heard of Cartesian coordinates. Thank Descartes. Very few contemporary philosophers hold the philosophical views Descartes held. His significance lays in the way he broke with prior tradition and the questions he raised in doing so. Descartes frames some of the big issues philosophers continue to work on today. Notable among these are the foundations of knowledge, the nature of mind, and the question of free will. We’ll look briefly at these three areas of influence before taking up a closer examination of Descartes’ philosophy through his Meditations of First Philosophy.

    To ask “How do we know?” is to ask for reasons that justify our belief in the things we think we know. Descartes’ Meditations provide a classic example of the epistemological project of providing systematic justification for the things we take ourselves to know, and this remains a central endeavor in epistemology. This project carries with it the significant risk of finding that we lack justification for things we think we know. This is the problem of skepticism. Skepticism is the view that we can’t know. Skepticism comes in many forms depending on just what we doubt we can know. While Descartes hoped to provide solid justification for many of his beliefs, his project of providing a rational reconstruction of knowledge fails at a key point early on. The unintended result of his epistemological project is known as the problem of Cartesian skepticism. We will explain this problem a bit later in this chapter.

    Another area where Descartes has been influential is in the philosophy of mind. Descartes defends a metaphysical view known as dualism that remains popular among many religious believers. According to this view, the world is made up of two fundamentally different kinds of substance, matter and spirit (or mind). Material stuff occupies space and time and is subject to strictly deterministic laws of nature. But spiritual things, minds, are immaterial, exist eternally, and have free will. If dualism reminds you of Plato’s theory of the Forms, this would not be accidental. Descartes thinks his rationalist philosophy validates Catholic doctrine and this in turn was highly influenced by Plato through St. Augustine.

    The intractable problem for Descartes’ dualism is that if mind and matter are so different in nature, then it is hard to see how they could interact at all. And yet when I look out the window, an image of trees and sky affects my mind. When I will to go for a walk, my material body does so under the influence of my mind. This problem of mind-body interaction was famously and forcefully raised by one of the all too rare female philosophers of the time, princess Elisabeth of Bohemia.

    A whole branch of philosophy, the philosophy of mind, is launched in the wake of problems for substance dualism. Today, the philosophy of mind is merging with neuroscience, cognitive psychology, and information science to create a new science of mind. We are rapidly learning how material brains realize the processes of thought. Once again, Descartes has failed in a most fruitful way. We also see how undeserved philosophy’s reputation for failing to answer its questions is. While many distinctively philosophical issues concerning the mind remain, the credit for progress will go largely to the newly minted science of mind. The history of philosophy nicely illustrates how parenthood can be such worthwhile but thankless work. As soon as you produce something of real value, it takes credit for itself. Later in a chapter on the philosophy of mind we will examine some developments in this area since Descartes and get acquainted with a few of its contemporary issues including the nature of consciousness.

    The final big issue that Descartes brought enduring attention to is the problem of free will. We all have the subjective sense that when we choose something we have acted freely or autonomously. We think that we made a choice and we could have made a different choice. The matter was entirely up to us and independent of outside considerations. Advertisers count on us taking complete credit and responsibility for our choices even as they very effectively go about influencing our choices. Is this freedom we have a subjective sense of genuine or illusory? How could we live in a world of causes and effects and yet will and act independent of these? And what are the ramifications for personal responsibility? This is difficult nest of problems that continues to interest contemporary philosophers.

    Descartes’ is also a scientific revolution figure. He flourished after Galileo and Copernicus and just a generation before Newton. The idea of the physical world operating like a clockwork mechanism according to strict physical laws is coming into vogue. Determinism is the view that all physical events are fully determined by prior causal factors in accordance with strict mechanistic natural laws. Part of Descartes’ motivation for taking mind and matter to be fundamentally different substances is to grant the pervasive presence of causation in the material realm while preserving a place for free will in the realm of mind or spirit. This compromise ultimately doesn’t work out so well. If every event in the material realm is causally determined by prior events and the laws of nature, this would include the motions of our physical bodies. But if these are causally determined, then there doesn’t appear to be any entering wedge for our mental free will to have any influence over out bodily movements.

    Now we will turn to Descartes’ Meditations and examine how he comes to the positions just outlined. Here is a link to several of Descartes’ writings including Meditations on First Philosophy:


    The Meditations
    Descartes project in his meditations is to carry out a rational reconstruction of knowledge. Descartes is living during an intellectually vibrant time and he is troubled by the lack of certainty. With the Protestant Reformation challenging the doctrines of the Catholic Church, and scientific thinkers like Galileo and Copernicus applying the empirical methods Aristotle recommends to the end of challenging the scientific views handed down from Aristotle, the credibility of authority was challenged on multiple fronts. So Descartes sets out to determine what can be known with certainty without relying on any authority, and then to see what knowledge can be securely justified based on that foundation.

    In the first meditation we are introduced to Descartes’ method of doubt. According to this method, Descartes goes through all of his beliefs, not individually but by categories, and asks whether there is any possible way that beliefs of this or that type can be mistaken. If so, they must be set aside as doubtable. Many of these beliefs may ultimately be redeemed as knowledge, but they cannot serve as part of the secure foundation of indubitable beliefs from which his rational reconstruction of knowledge proceeds. Empirical beliefs, things that we believe based on the evidence of our senses, are set aside first. Our senses sometimes deceive us, as when an oar appears bent in water or a stranger in a crowd appears to be a friend. It won’t do to say that we can reliably diagnose these cases and correct for mistaken appearances though because we also have experiences just like seemingly reliable sense experiences that are anything but in the case of dreams. How can we be certain that any of our seeming sense experiences of the external world aren’t in fact dreams? How can we be certain that our whole life isn’t a dream?

    So sense experience is set to the side as uncertain and insufficient for justifying knowledge. Descartes then considers things we might know for certain by the light of reason, like mathematical claims. I seem to be about as certain in my belief that 2+2=4 as I can be about anything. Is there any possible way I could be mistaken? Descartes here imagines a powerful demon that could deceive me into always thinking that 2+2=4 when in fact this is not true. Is this a genuine possibility? Descartes allows that it is and considers all such knowledge had through reason doubtable as well.

    Does anything remain? Are there any beliefs that can’t be doubted, even given the hypothesis of a powerful evil deceiver? Descartes does find at least one. Even an evil deceiver could not deceive Descartes about his belief that he thinks. At least this belief is completely immune from doubt, because Descartes would have to be thinking in order for the evil deceiver to deceive him. In fact there is a larger class of beliefs about the content of one’s own mind that can be defended as indubitable even in the face of the evil deceiver hypothesis. When I look at the grey wall behind my desk I form a belief about the external world; that I am facing a grey wall. I might be wrong about this. I might be dreaming or deceived by an evil deceiver. But I also form another belief about the content of my experience. I form the belief that I am having a visual experience of greyness. This belief about the content of my sense experience may yet be indubitable. For how could the evil deceiver trick me into thinking that I am having such an experience without in fact giving me that experience? So perhaps we can identify a broader class of beliefs that are genuinely indubitable. These are our beliefs about the contents of our own mind. We couldn’t be wrong about these because we have immediate access to them and not even an evil deceiver could misdirect us.

    The problem Descartes faces at this point is how to justify his beliefs about the external world based on the very narrow foundation of his indubitable beliefs about the contents of his own mind. And this brings us to one of the more famous arguments in philosophy: Descartes’ “Cogito Ergo Sum” or “I think, therefore I exist.” Descartes argues that if he knows with certainty that he thinks, then he can know with certainty that he exists as a thinking being. Many philosophers since then have worried about the validity of this inference. Perhaps all we are entitled to infer is that there is thinking going on and we move beyond our indubitable foundation when we attribute that thinking to an existing subject (the “I” in “I exist”). There are issues to explore here. But bigger problems await Descartes, so we will just note this one and let it pass.

    So far Descartes has only adequately justified his beliefs about the contents of his own mind and his own existence as a thinking being. Knowledge about any external reality or even truths of reason like 2+2=4 remain in need of justification. To overcome skepticism about these matters, Descartes sets out to prove that God exists and is not an evil deceiver. Once the evil deceiver hypothesis is in check, Truths of reason and perhaps others may be yet be knowable. Not any argument for God’s existence and good nature will do, though. The trick for Descartes’ project of a rational reconstruction of knowledge is to prove the existence of a good God by reasoning only from those beliefs that he has identified as indubitable and foundational.

    Descartes argument for the existence of a good God goes roughly as follows:

    1. I find in my mind the idea of a perfect being.

    2. The cause of my idea of a perfect being must have at least as much perfection and reality as I find in the idea.

    3. I am not that perfect.

    4. Nothing other than a good and perfect God could be the cause of my idea of a perfect being.

    5. So, a good and perfect God must exist.

    This argument simplifies the rather involved reasoning Descartes goes through in the Meditations. But it will do for diagnosing the fatal flaw in Descartes’ reasoning. Let us grant the validity of the argument and consider the truth of its premises. Keep in mind that to accord with the method Descartes has set for himself in carrying out a rational reconstruction of well grounded certain knowledge, all of the premises of this argument must be indubitable and foundational. Being a belief about the contents of his own mind, we can grant the certain truth of premise one. Though it is not as clear, premise three might arguably count as a foundational belief about the contents of Descartes’ own mind. An evil deceiver, being evil, would lack perfection found in Descartes’ idea of a perfect being. So as powerful as such a being could be, the cause of Descartes’ idea of a perfect being must be more perfect than any evil deceiver. Perhaps any being so perfect would have to be a good God.

    But the fatal flaw for Descartes’ rational reconstruction of knowledge is the second premise. What are our grounds for thinking that the cause of something must have at least as much perfection as its effect? The idea of degrees of perfection and the notion that the less perfect can only be explained in terms of the more perfect is an idea that we find in Plato’s theory of forms. It will strike many of us as implausible or even incomprehensible. Just what is perfection supposed to mean here? And even once we’ve spelled this out, why think causes must be more perfect? It seems not at all uncommon for less perfect things to give rise to more perfect things (just consider my son, for instance). In any case, whether the second premise can be explained and defended at all, the fatal flaw for Descartes’ project is that it is not foundational. It is not an indubitable belief about the contents of Descartes’ own mind, but rather a substantive belief about how things are beyond the bounds of Descartes’ own mind. So Descartes’ attempt to provide a rational justification for a substantive body of knowledge leaves us with an enduring skeptical problem. All we have immediate intellectual access to is the contents of our own minds. How can we ever have knowledge of anything beyond the contents of our own mind based on this? This is the problem of Cartesian skepticism.

    Having diagnosed the fatal flaw in Descartes’ project, we should briefly consider how his rational reconstruction of knowledge was to go from there. Given knowledge of God’s existence and good nature, we would appeal to this to assure the reliability of knowledge had through reason and later also through the senses. God being the most perfect and good being would rule out the possibility of interference by an evil deceiver. We might still make mistakes in reasoning or be misinformed by the senses. But this would be due to our failure to use these faculties correctly. A good God, however, would not equip us with faculties that could not be trusted to justify our beliefs if used properly. This is a very cursory summary of the later stages of Descartes’ attempted rational reconstruction of knowledge in his Meditations. But it will suffice for our purposes.


    The mind-body problem

    Descartes is a substance dualist. This is the metaphysical view that the world is made up of two fundamentally different kinds of substance: matter and spirit (or mind). In the Second Meditation Descartes motivates this view by arguing that there are distinguishing differences between the mind and the body. In particular, I can doubt the existence of my body but I can’t doubt the existence of my mind. Is this a difference that justifies denying that the mind is in some sense identifiable with the body? If something is true of one thing and not of another, then we have conclusive grounds for thinking they are not one and the same thing. So if my favorite bike is red but the bike in my office is yellow, then the bike in my office is not identical to my favorite bike. Does this straightforward line of reasoning apply to the case of the mind and the body? The existence of my body is dubitable, but the existence of my mind is indubitable. Descartes would count this as a reason for denying that my mind is identical with my body. But consider this analogous argument:

    1. Mark Twain is such that Joe thinks he is the author of Huckleberry Finn.

    2. Samuel Clemens is not such that Joe thinks he is the author of Huckleberry Finn.

    3. So, Mark Twain is not identical to Samuel Clemens.

    Clearly the conclusion that Mark Twain and Samuel Clemens are not one and the same person does not follow in this case. As the Mark Twain argument looks like it is closely analogous to Descartes’ argument for the non-identity of the mind and the body, it looks like Descartes’argument is not valid. The problem here is that the premises of both arguments concern someone’s mental states about something. In Descartes’ argument we have premises about what he can or can’t doubt. In the Mark Twain argument we have premises about what Joe does or doesn’t believe. But people can fail to recognize true identity claims. Joe doesn’t know that Mark Twain just is Samuel Clemens. Because of this, Joe can believe one thing about Mark Twain and something different about Samuel Clemens. But this doesn’t show that Mark Twain and Samuel Clemens aren’t identical. Likewise, it may be that the mind is identical with the body or some part of it, but since Descartes doesn’t know this, he can believe one thing about the body (that its existence is doubtable) and something else about the mind (that its existence is not doubtable).

    So far we have just offered a critical evaluation of one of Descartes’ arguments for mind/body dualism. Now we will consider a serious problem for the view. When Descartes’ considers how the substance of mind and body differ, he offers a view that should sound familiar from popular religious belief. On this view, the body is a physical object that exists in space and time and is subject to the laws of nature. The mind, being spiritual in nature, exists eternally in an abstract realm rather than existing in the physical realm of space and time. Further, the mind is not bound by mechanistic laws of nature, but it has free will that allows it to will or not will to do one thing or another. Descartes was both a believer in Catholicism and an active participant in the scientific revolution. He was among those who were developing a view of the natural world in which events occur in accordance with strict law-like regularities. A view of the natural world as functioning like a predictable clockwork mechanism was on the rise. And yet Descartes’Christian theology held that as a person created in the image of a divine being, he had free will through which he might choose to do one thing or another, perhaps most notably, to choose to accept the Catholic faith as true and be saved or not. His philosophical view is an attempt to reconcile these conflicting scientific and theological perspectives.

    An unfortunate fact of history is that women in Descartes’ time were rarely given a thorough education or allowed to participate fully in intellectual life. A notable exception is the case of the Princess Elisabeth of Bohemia. And she was among the first to notice serious difficulties in the substance dualism that Descartes advocated. The central problem has to do with mind-body interaction. Clearly things going on in the physical realm have an influence on the mind. Light reflecting off clouds and trees cause me to have the mental perception of a sunset. And likewise, mental phenomenon cause things to happen in the physical world. When I mentally will to preserve the image of the sunset in a picture, my body causes things to happen in the law governed physical realm. I reach for my camera. But how can a non-physical soul be affected by or effect events in the physical realm? If events in the physical realm are all transfers of physical energy happening at specific places and times, how can it be that the non-physical mind has any role to play in this? The problem gets all the more difficult when we take the physical world to be deterministic, governed by laws where each event is determined to happen by prior events in conjunction with mechanistic laws. Determinism in the physical realm would appear to leave no room for the non-physical mind to influence events at all. Contemporary philosophers who study the nature of the mind generally take these problems to be intractable and to constitute decisive objections to Descartes’ substance dualism. More recent philosophy of mind has been mainly taken the mind to be physical. And philosophers, along with neuroscientists and cognitive psychologists, are making tremendous progress in understanding how mental phenomenon can be understood in biological, physical terms. When we take a closer look at more recent developments in the philosophy of mind, however, we will find some arguments for denying that some mental properties, consciousness in particular, can ever be identified with purely physical properties or processes.


    Study Questions for Meditations 1-3

    1. Explain Descartes’ method of doubt. What is Descartes’ purpose in exercising this method?

    2. Why can’t Descartes be certain about beliefs he acquires through the evidence of the senses?

    3. Why can’t Descartes be certain about mathematical beliefs, like the belief that 2+2=4?

    4. What belief(s) does Descartes ultimately identify as indubitable?

    5. Why can’t an evil deceiver deceive Descartes about his belief that he thinks?

    6. How does Descartes build up from the foundation of indubitable beliefs?

    7. How does Descartes argue for the existence of God?

    8. Given the existence of God, how does Descartes justify his beliefs based on reason and on the senses?

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