Lecture Notes, UC Davis Philosophy 102, Theory of Knowledge


G. J. Mattey, Senior Lecturer

René Descartes

Text: Meditations One through Three (1741)

One of the most influential figures in modern theory of knowledge is the seventeeth century French René Descartes. His main project was to develop and defend a mathematically-based account of the natural world. One of the greatest threats to this new science was posed by Pyrrhoninan skepticism, which had been revived in the sixteenth century, especially by Michel de Montaigne. Skepticism was also a threat to religious belief, and Descartes wished to defend the Catholic faith against "the errors of the atheistic skeptics" (Replies to Seventh Objections to the Meditations, Reply 4).

The Skeptical Threat

Descartes considered "hallmark of the skeptics" to be the error of "excessive doubt" (Reply 4 to Seventh Objections). The skeptics "go beyond all the boundaries of doubt." We could regard them merely as "desperate lost souls," but they should really be refuted.

[W]e should not suppose that skeptical philosophy is extinct. It is vigorously alive today, and almost all those who regard themselves as more intellectually gifted than others, and who find nothing to satisfy them in philosophy as it is ordinarily practiced, take refuge in skepticism because they cannot see any alternative with greater claims to truth. (Reply 4 to Seventh Objections)
This description is a fair representation of Pyrrhonian skepticism: an inability to choose among existing accounts of the nature of things leads to widespread suspension of judgment.

In the area of "natural philosophy" (which we now call "natural science,") there was a real opening for skepticism. The orthodox account of nature, based on the writings of Aristotle, were under fierce attack in the seventeenth century, with Descartes as one of the chief assailants. Since neither of the rival accounts of nature was agreed upon, the problem of the standard arises. Descartes was trying to justify a standard which would favor his "modern" approach to nature over the "ancient" one.

In the area of religion, the Protestant Reformation had upset the agreement among Western Europeans on matters of religion or on the standard for establishing them. All agreed that scripture was authoritative, but there was a fundamental disagreement about how it should be interpreted. The Roman Catholics used tradition and the authority of the Church as a standard of truth. Any disagreement over these standards would have to be settled by appeal to the very same standards: what the Pyrrhonians called the "reciprocal mode." Luther replaced these standards of interpretation with individual conscience. To this day, there is no agreement over the standard of interpretation of scripture.

Montaigne followed the Pyrrhonian skeptics, who said that when a standard is lacking, one should follow local customs. In France meant accepting Catholicism. This "fideistic" response to skepticism gained some popularity in Europe. (For more on this issue, see Richard Popkin, The History of Scepticism from Savonarola to Bayle.)

But this Pyrrhonian fideism hardly the only possible response to lack of agreement about religious issues and about the standard for establishing their truth. One could simply suspend judgment about them because, unlike the questions "whether he has a head, or whether two and three make five," religious questions and no answer that appears to be true. According to Descartes, the skeptics

do not see the existence of God and the immortality of the human mind as having the same appearance of truth, and hence they are unwilling to treat these claims as true for practical purposes unless and until they have seen them proved by means of arguments more reliable than any of those which lead them to accept whatever is apparently true. (Rebuttal to Reply 4 to Comments on Section 9 of Seventh Objections)
Descartes saw it as his task to provide "a reliable proof of these matters, and this is something that no one, as far as I know, has done before" (Rebuttal to Reply 4 to Comments on Section 9 of Seventh Objections). He would then use his establishment of God's existence to overcome skepticism in natural philosophy.

So does one go about giving a reliable proof that God exists and that the human mind is immortal? Such a proof would have to be based on something a skeptic would accept as having the appearance of truth. Specifically, it would be something for which there is no reason that would induce any doubt. Such a truth would serve as a "foundation" from which Descartes could build his "reliable proof."

Finding the Foundation

One of Descartes's most distinctive contributions to theory of knowledge is the method he proposed for finding the indubitable foundational truths. He proposed to reject as false any of his current beliefs that left any room for rational doubt. If he could discover any beliefs which survived this most stringent test, the would have the foundation he was looking for.

Here is an analogy he used to describe this "method of doubt" in the fact of a critic he accused of misunderstanding him.

Supppose he [the critic] had a basket full of apples and, being worried that some of the apples were rotten, wanted to take out the rotten ones to prevent the rot spreading. How would he proceed? Would he not begin by tipping the whole lot out of the basket? And would not the next step be to cast his eye over each apple in turn and pick up and put back in the basket only those he saw to be sound, leaving the others? (Comments on Section 2, Seventh Objections)
Descartes thought that what induced error in our belief is our experience as children, where we blindly follow the evidence of the senses. The most pervasive error in our belief regards physical objects, which we think really resemble the way they appear. The Pyrrhonians would agree on the failure of sense-experience to settle disputes about the way things really are. But Descartes thought that, if he started over from new foundations, he would be able to overcome all doubts on the matter.

What May Be Doubted

Descartes proceeded by considering large classes of beliefs that he held at the beginning of his investigation. If he could find a characteristic feature of a kind of belief that opened the door for rational doubt, then he would withhold his assent (for the time being) to any beliefs of that kind. So with respect to objects that are distant or small, he noted that we frequently make false judgments. The stock example is one from Sextus, that a round tower looks square at a distance. Following his method, Descartes undertook to suppress his beliefs about distant and small objects.

It might seem that nearby and normally-sized objects are not prone to the same kind of error. After all, we correct our judgment about the shape of a distant tower by viewing it up close. But this correction presumes that one is actually perceiving the tower with one's eyes. There is reason to doubt, in any given case, that one is actually doing this, because one can be dreaming that this is what one is doing. Descartes notes that there have been times when he was asleep and dreaming that he was seated by the fire in his gown, when in fact he was asleep unclothed in his bed. So long as there is some possibility of error in determining that one is awake, there is rational doubt about not only the properties, but even the existence, of we believe to be objects detected by the bodily senses.

This led Descartes to try to suspend his beliefs in the existence of every physical object, including his own body. Such a wholesale rejection of these deeply ingrained beliefs is something that the Pyrrhonian skeptics never as much as suggested. Their method did not require them to doubt the appearances. But Descartes's project was not to give a practical presciption for what to believe. He wanted to find indubitable beliefs that would serve as foundations of knowledge, and to do so, he was willing to risk never recovering his beliefs in the existence of bodies.

Philosophers since Descartes have mostly decided that Descartes's risk was too great. Thomas Reid, writing about a hundred years later, characterized Descartes as deliberately marching into a coal pit when there was no need to do so. It should be noted that Descartes's march was only for the purposes of finding an indubitable foundation in order to refute skepticism. It was not supposed to show that the possiblity of dreaming makes it unreasonable to believe in the existence of bodies.

Supposing the worst-case scenario, that all the images before the mind are the products of dreams, it still seems that there remains room for beliefs. The images themselves have their properties, such as color and shape, so perhaps we can have some general knowledge that at least applies to the colors, shapes, etc. in a dream-world.

Descartes found that even here there is room for rational doubt. The grounds for doubt are different from those in the first two classes of beliefs, however. Descartes does not appeal to any particular errors that he has made in the past regarding the general matters in question. Rather, he notes that he may be in error (without being able to detect that he is) due to some defect in his constitution.

Why would he suspect that his constitution might be defective? The answer can be put in terms of a dilemma. Either his constitution is the product of a perfect being or else it is the product of an inferior being. If it is the product of an inferior being, then there is the chance that he was produced with some imperfection, given the limitations of his cause. If his constitution is the product of a perfect being (God), then that being has the power to create him in such a way that he is deceived about what seems to be most evidently true.

Reclaiming Belief

The basic strategy in the rest of the Meditations is to prove that God exists and that God would not create him in such a way that he would be mistaken in what has the greatest appearance of truth. But there is some preliminary work that must be done before these points can be established. In particular, Descartes needed to find something that can be used as indubitable premises in the proof of God's existence.

The clue is to be found in the process of doubting itself. Suppose Descartes asks himself whether he can be in error about whether he exists. The answer is that he cannot, so long as he is actually contemplating the possibility of his not existing. So here is an indubitable truth, at least when I am contemplating my existence: "I exist." (This is known as Descartes's cogito, which is Latin for "I think." It indicates the fact that the act of contemplation is required for the certainty of "I exist.") Note that even the hypothesis that his constitution is defective does not threaten the truth of "I exist."

This belief does not seem to very promising as a foundation for other beliefs, at least when it is taken by itself. "I exist" is a very specific belief, and to establish God's existence, Descartes will have to appeal to more general beliefs. So Descartes tries to use his one indubitable belief as leverage to generate a class of beliefs on which he can rely.

He first investigates what is the "I" which exists when he contemplates its existence. He finds that it at least has the properties that are required for the self-contemplation involved in the method of doubt. So, he is something which doubts and believes, which assents or witholds assent, which wills, and so on. This is a very clever move, for it uses what is involved in doubting to establish what is true.

Two other activities of the "I" that Descartes discovers are imagining and sensing. These are not required for the method of doubt, so Descartes relies simply on the observation that his mind forms images by exercising its will (imagining) and without exercising its will (sensing). Let us call these the "imaginative functions" of the mind. He can hardly doubt that there are images in his mind, even if he can doubt that these images represent anything in reality apart from his mind.

In the rest of the second Meditation, Descartes compares the purely intellectual functions of the mind with the imaginative functions, in terms of how well they represent objects. He finds that the intellectual functions operate without limit, while the imaginative functions are limited to the images that are actually formed. For this reason, Descartes is often classified as a "rationalist."

The Standard of Truth

At the beginning of the third Meditation, Descartes takes stock of what he can indubitably believe, i.e., that he exists and that he engages in various mental operations. This is still not enough to generate a proof of God's existence, so he makes one more move--a critically important one--that will provide the missing ingredients.

He asks what it is about what he has discovered up until now that leads him to think that it is indubitable. His answer is that the objects of indubitable belief are perceived very clearly and very distinctly. He can find nothing else that could serve as a sign of its truth.

Certainly in this first knowledge here is nothing that assures me of its truth, excepting the clear and distinct preception of that which I state, which would not indeed suffice to assure me that what I say is true, if it could ever happen that a thing which I conceived so clearly and distinctly could be false; and accordingly it seems to me that I can already establish it as a general rule that all things which I perceive very clearly and very distinctly are true.
This is his key move: to adopt the clarity and distinctness of perception as a general standard of truth. (Note that by "perceive" here, Descartes means a purely intellectual act different from sense-perception, which he took to be a specialized kind of perception.)

With this standard in mind, Descartes revisits the beliefs that he has tried to hold in suspense until this point. He finds that he perceived nothing clearly and distinctly about physical bodies except for the fact "that the ideas or thoughts of these things were presented to my mind." So he cannot use his standard to say that bodies truly exist.

Next, he considers "anything very simple and easy in the sphere of arithmetic and geometry." These do seem to meet the standard of truth, and yet he had held them to be dubitable because of the possibility that "God might have endowed me with such a nature that I may have been deceived even concerning things which seemed to me to be the most manifest." This seems to pose a threat to the standard of truth: God could "cause me to err, even in matters in which I believe myself to have the best evidence."

Descartes tries to dispel this threat by a kind of cry of defiance. When he is considering those things he perceives very clearly and distinctly, he simply refuses to accept that he might be deceived about them.

And, on the other hand, always when I direct my attention to things which I believe myself to perceive very clearly, I am so persuaded of their truth that I let myself break out into words such as these: Let who will deceive me, He can never cause me to be nothing while I think that I am, or some day cause it to be true to say that I have never been, it being true now to say that I am, or that two and three make more or less than five, or any such thing in which I see such a manifest contradiction.
Despite Descartes's display of confidence in the moment of contemplation, he acknowledges that at other times a "slight, and so to speak metaphysical" doubt remains, since he has no reason to believe that there is a deceiving God (or even that there is a God with the power to deceive him). He would go on to try to prove that God exists and is not a deceiver.

Descartes's earliest commentators spotted a problem with this approach. Descartes asserts that "without a knowledge of these two truths, I do not see that I can ever be certain of anything." But how is he supposed to establish a knowledge that God exists and is no deceiver without basing it on premises that are already certain? It seems that he needs certainty to prove God's existence and a proof of God's existence to attain certainty. (This has come to be known as the "Cartesian circle.")


There are several important issues raised by Descartes that remain controversial in the theory of knowledge. The first issue is whether and how skepticism might be refuted. Nobody takes Descartes's approach any more, but other strategies have been advanced. A related issue is whether there is any reason even to try to refute skepticism.

One of the most important moves Descartes made was to try to use the internal resources he discovered in his own mind as the basis of all other knowledge. Many philosophers have questioned this first-person starting-point for theory of knowledge. Finally (for our purposes), many philosophers have charged that Descartes's demand for knowledge to be indubitable (or at least to rest on indubitable foundations) sets the bar too high and makes the skeptic's job too easy.

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