Study Guide: René Descartes



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Study Guide: René Descartes
Descartes was a rationalist: He believed that at least some synthetic propositions could be known a priori (that is, without being based on experience). For an example of such a proposition, see the Wax Example section below.
Descartes wanted to apply the methodology of mathematics to science and philosophy, and thus get mathematical precision and certainty in these other areas.

  • Avoid relying upon observations (which could be mistaken and thus introduce uncertainty). Instead, use only intuition and deduction.

  • Intuition: Intellectually grasping a truth that is so clear and fundamental that it is impossible for one to be mistaken.

  • Deduction: Beginning only with facts one knows with complete certainty, reasoning to reach further facts that are known with certainty.

    • Any belief of which one is not completely certain cannot qualify as knowledge.

Descartes’s Meditations on First Philosophy are intended to show that we can be absolutely certain of the truth of many of our ordinary beliefs. Descartes is presenting this as an argument against skepticism.


  • Skepticism: Any claim that some body of beliefs do not qualify as knowledge. Examples:

    • Skepticism about the future: Although we have many beliefs about what will happen in the future (and may even say that we know that certain things will happen), this sort of skeptic says that these beliefs do not qualify as knowledge. There are many ways in which our beliefs might turn out to be false.

    • Skepticism about religion: Although we have many religious beliefs (and even claim that we know that certain religious claims are true), this sort of skeptic argues that these beliefs do not qualify as knowledge. Again, there are many ways in which these beliefs might turn out to be false.

  • Skepticism often involves presenting skeptical alternatives for the beliefs it challenges.

    • A skeptical alternative is a situation in which (a) our evidence for the belief in question is exactly the same, and (b) the belief is false.

    • E.g., You visit the zoo and believe that there is a zebra in front of you. The evidence consists of your visual experience (seeing a striped, roughly horse-like animal, as well as seeing the sign on the fence that says “zebra”). A skeptical alternative is a situation in which you have that same evidence (i.e., things look exactly the same to you), but the belief is false. For instance, it could be that the animals you see are not zebras, but mules painted with stripes.

    • On Descartes’s view, a belief qualifies as knowledge, your evidence for it would have to be so good that no skeptical alternatives are possible. That is, your evidence could not be explained in any situation in which that belief were false.

1st Meditation: Descartes presents the skeptical challenge that he will be arguing against.

  • He presents a form of global skepticism; it challenges all of our beliefs, saying that none of them qualify as knowledge. For every sort of belief, Descartes presents skeptical alternatives. Examples:

    • Perceptual beliefs: Could be illusions, hallucinations, or dreams.

    • Memories: The past may have been different than you remember.

    • Calculations: You may have made an error in your calculations.

    • Simple beliefs about arithmetic or logic: You may have a feeling of certainty when considering false beliefs.

  • Descartes has a general purpose skeptical alternative: It could be that you are being deceived by evil demons. (This is the 17th-century version of the brain-in-a-vat case or a Matrix-type situation.) If we were in such a situation, our evidence would all be exactly the same, but our beliefs would be false.

  • Note that Descartes is NOT a skeptic, even though it may seem otherwise in this meditation. He is just presenting the position he will be arguing against.

  • The criterion of truth Descartes is using here is the indubitability criterion: A belief will be accepted as true only if it cannot be doubted. Beliefs can be doubted whenever there are skeptical alternatives for that belief (i.e., when there is a possible situation in which our evidence is the same but the belief is false).


2nd Meditation: Using his indubitability criterion of truth, Descartes finds a belief that qualifies as true (i.e., that cannot be doubted, that qualifies as knowledge): The existence of his own mind.

  • He arrives at this by the famous “I think, therefore I am” (Cogito ergo sum). The very fact that he is capable of doubting proves that there must be a mind doing that doubting.

  • Although Descartes thinks there are no skeptical alternatives for this belief, Hume would later argue that there are. For instance, it could be that mental states (such as doubts) can exist independently, without being part of a mind. If that were the case, the experience of doubting would not guarantee the existence of a mind. Instead, it could be that the experience of doubting exists without being part of a mind.

The 2nd Meditation also includes the Wax Example, an argument against empiricism. Descartes describes seeing a piece of wax at two different times, but the wax has completely different sets of properties each time. Descartes then comes to believe that the wax could have infinitely many different sets of properties. This, he claims, is a synthetic proposition known a priori. Although experience (the 2 observations of the wax) is the occasion for coming to believe this, Descartes says that this is not the basis for this belief. To believe this a posteriori, he argues, would require observing the wax infinitely many times, with a different set of properties each time.

  • The wax example is said to provide an example of a synthetic proposition that is known a priori. The rationalists say this is possible, and the empiricists don’t, so this seems to disprove empiricism.

    • In response, the empiricists argue that this belief is known a posteriori, and that Descartes is conveniently forgetting about all of the other experiences that went into forming the belief in question.

  • The wax example also supports the “pincushion” view of physical objects (later adopted by Locke and criticized by Berkeley). This states that a physical object is more than just the sum of its properties; the properties are “stuck onto” an unperceived something which is the object itself.

In the 2nd Meditation Descartes also distinguishes between imagination (thought involving mental images) and understanding (a non-pictorial grasp of something).
3rd Meditation: This meditation begins with the egocentric predicament: Descartes has proved that his own mind exists, but how can he prove the truth of any beliefs about the external world? To address this problem, Descartes replaces the indubitability criterion of truth with the clearness and distinctness criterion of truth: A belief will be accepted as true if one has a clear and distinct idea of it.

  • An idea is clear and distinct if there is nothing hidden or unclear in it, it can be entirely present to one’s mind at the same time, and all parts of the idea are vivid.

The new clearness and distinctness criterion cannot be used until Descartes has proven that clear and distinct ideas are guaranteed to be true. After all, it would seem possible to have a clear and distinct idea of something false. To show that the new criterion is reliable, Descartes will (a) prove that god exists, and (b) show that god would not allow us to be deceived about clear and distinct ideas.

Proving the Existence of God: Descartes’s argument relies upon a scale of reality and a causal principle that he gets from medieval philosophy. Causal Principle: The cause of x must have at least as much reality as x. This principle presupposes that there is a scale of reality, with some things being more real than others. An object is higher on this scale of reality if it is more powerful or more perfect. Thus, god is at the top of the scale. Note that ideas are also on this scale. An idea is at the same level as the object of which it is an idea.

  • Descartes begins by noting that we have an idea of god.

  • Because god is at the top of the scale of reality, the idea of god must also be at the top of the scale.

  • The causal principle states that this idea of god must have been caused by something at least as real as the idea itself.

  • The only thing that could possibly have caused my idea of god is god. In order to have caused my idea, god must exist.

  • God is perfect, and thus will never deceive us. (Deception is an imperfection.)

  • If god allows us to be mistaken about clear and distinct ideas (or about illusions or hallucinations), then god would be a deceiver. We know this is impossible.

  • Thus, our clear and distinct ideas must be true, so the clearness and distinctness criterion of truth is reliable. (Also, our senses must be generally reliable, too.)

The Cartesian Circle: This argument is circular; it presupposes the truth of what it is supposed to be proving, and thus is not convincing. Until Descartes has established the reliability of his new clearness and distinctness criterion of truth, he is permitted to use only the indubitability criterion. The argument for god’s existence is an essential part of proving the reliability of the clearness and distinctness criterion, but Descartes assumes that this new criterion is reliable in the course of that argument.

  • The argument for god’s existence could not work if Descartes used only the indubitability criterion of truth. We could certainly doubt the scale of reality and the causal principle.

  • In defending the scale of reality and the causal principle, Descartes says they are true because we have a clear and distinct idea of them, and such ideas are always true. Thus, he is assuming the truth of this criterion in order to show that it is true. This reasoning is circular.


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