The Method of Doubt

Descartes adopts the method of doubt—a systematic examination of all of his beliefs to see if he can find an absolutely secure foundation for knowledge. He compares this process to culling apples in a cart. A systematic way is to empty the cart, examine each apple, and put the good ones back. Descartes is going to empty his mind of beliefs and put true beliefs back when he is finished.[1]

Could It All Be a Dream?

Descartes notes that wakeful experiences may be indistinguishable from those of dreaming. But if he cannot distinguish dreaming from being awake, for all he knows, life could be a dream and there may be no external world. If he is dreaming, his senses cannot provide a foundation for knowledge. He must look for something else that can.

Descartes turns to truths graspable by the mind itself. Like Plato, he sees mathematical truths as being independent of sense perception. Could mathematics be the foundation of knowledge he is looking for? Suppose Descartes is certain that 2 + 2 = 4. But what if he just has a strong feeling of being certain while he is, in fact, mistaken? Perhaps a demon—an evil deceiver—is creating a strong feeling of certainty in his mind. He cannot rule out an evil deceiver and so cannot take mathematics as a foundation for knowledge. He must look elsewhere.

He sees clearly that even if he is deceived about the world as given to him by his senses, and is also deceived about mathematics, he must exist. This is a logical point. To be deceived, he must exist. No demon can fool him about this. Deception requires thinkers.

Descartes concludes that his essence is to be “a thing which doubts, understands, conceives, affirms, denies, wills, refuses, and which also imagines and feels.” As for Plato, our essential nature is contemplation. The body does not belong to our essential nature. We can imagine we do not have our body. But we cannot imagine that we exist if we have no thoughts (no conscious experience of mental contents).

Descartes turns again to the external world and material bodies. By bodies, he means any physical thing, including his own body. Descartes is curious about the sense he has of being situated in a world of bodies. Why does he naturally think of the world as being made up of bodies? In asking this question, he makes a simple experiment.

Descartes takes a piece of wax and notes how it appears solid, with a shape, color, and smell. When he moves it near a candle flame, it softens and then melts. From what his senses tell him, the wax is now very different. How can this be? Why does he think it is the same wax? He believes he has the innate idea of identity. An innate idea does not come from experience. No matter how often he melts a piece of wax, he cannot understand that it is the same unless he already understands what it means to retain an identity.

The wax example suggests he can grasp material bodies without perceptual qualities. The wax was the same material body, even though his senses told him everything had changed. Material bodies can be grasped as having extension. Extension is the only constant of material bodies; they always occupy space. As the essence of the mind is to think, so the essence of body is to be extended. It is a geometrical way of thinking about bodies as existing without perceptual qualities. When Descartes thinks about bodies in this way, he thinks of them as being of bodily or material substance. Substances exist by virtue of their essences. So all bodies are said to be of material substance.

He also thinks the mind is a substance—a mental substance. At this point in his meditations, Descartes does not know that material bodies exist. He knows only that he has certain clear and distinct ideas about them. Again, however, he could be deluded by an evil demon, so perhaps there are no extended things. It could be that the world is an illusion created by a demon. How can he get out of his mind to the external world? How can he prove that the external world exists? His foundation for science is at stake.

Descartes appeals to God. He has the innate idea of God as a perfect being. A being that didn’t exist eternally would not be a perfect being. Hence, God must exist. He thinks that this can be understood by the analogy that the angles of a triangle must sum to 180 degrees. It is part of the idea of a triangle that the angles add up to 180 degrees, and it is part of the idea of a perfect being to exist.

God would not allow that Descartes was deceived about mathematics, because it is not in God’s nondeceitful nature. What about the external world? Could it be an illusion? God would not allow this either; it too would constitute a deception. Descartes is back in the world. Now, however, he knows it mathematically as a world of geometric bodies.

What about our sensory experiences? The world of human experience doesn’t seem to be one of extended geometric shapes in space. It seems rich, colorful, and lively. The enjoyment of a glass of red wine or of skiing through fresh powder snow has little to do with mathematical geometries. How can Descartes account for subjective experiences? The universe, as he thinks of it, is full of geometric bodies pushing right up against each other. Our body is no exception; it is in communication with other bodies, always pushing against other things or being pushed against. Descartes believes that our sensory experience results from mechanical interaction between our body and things in the external world, plus some translation between our body and our mind. He tells us that this translation takes place in the brain’s pineal gland, where mind and body connect. When he moves his hand to reach an apple, his mind moves his arm through this gland. When he touches the apple, sensory signals from his hand travel through his body and the pineal gland to the mind, and he experiences touching it. Descartes decided on the pineal gland because it is a centrally located anatomical structure and it is not part of a pair. Most brain structures come in pairs (one for each half of the brain), and so the pineal gland was a reasonable choice.[2] But he could not explain how the mind and body interact. This problem has become known as the mind-body problem, and the Western intellectual tradition continues to struggle with it.

Descartes thought about mind and body as separate substances, utterly different from each other. The mind is a thinking substance, while the body is a material substance. How, then, can they interact? Let us look at the properties Descartes ascribed to mind and body. On the one hand, the mind is a nonextended thinking thing. The body, on the other hand, is essentially extended. Material bodies can neither think nor be conscious, nor are they free like the mind. Bodies move as determined by mechanical interaction with other bodies. They have shape, location, and mass, but the mind lacks those attributes.[3]

Descartes’s mathematical physics also led him to say that bodies must be infinitely divisible. He thinks of bodies as geometrical objects that can be divided into ever smaller parts. The mind, however, is indivisible. It cannot be divided, because it is not a spatially existing object. Since the mind cannot be divided into parts it is, according to Descartes, indestructible and eternal. Bodies, on the other hand, exist temporally and will disintegrate.

We commonly think of ourselves in Cartesian ways. Our mind is seen as weightless, without mass and spatial dimensions. The mind thinks, feels, wills, and experiences, while our body is understood physically. It is even possible to acquire new parts—such as hearts, joints, and teeth—for our body. Conversely, one cannot obtain new parts for the mind or otherwise repair the mind in a similar fashion. The mind is a nonmaterial unity. Now that we have seen how Descartes conceives of mind and body, we can take a closer look at the problems raised by his dualism.

  • [1] Descartes introduces his method of doubt in the first meditation of his Meditations on FirstPhilosophy (Descartes and Moriarty 2008).
  • [2] Little did he know that the pineal gland is divided inside into two structures at the micro level.
  • [3] One could say that the mind or soul in Descartes’s view ought to be thought of as being where ithooks up to the pineal gland. However, at the same time, we are not to think of the soul as a spatialphenomenon. However we read Descartes, we are not going to get a fully coherent picture. Theproblem lies with his dualist position.
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