Descartes' argument for Dualism

Descartes wanted to believe that the mind was something special and separate from the matter that makes up our bodies. After all, if the mind is of the same stuff as the rest of the universe, how can you defend the idea of free will from charges of determinism? So he came up with some arguments supporting dualism, the world view that the mind and the body are intrinsically different from each other. Here is his first attempt:

(1) I am not dubitable -- See The method of doubt.
(2) My body is dubitable
(3) I and my body do not have exactly the same Intrinsic properties
(4) If I am the same thing as my body, then I and my body have exactly the same intrinsic properties.
Therefore (5) I am not my body

The problem with this argument is that doubt is not an intrinsic property. It is projected by the doubter. Therefor this argument is not sound.

Next Descartes tried replacing dubitable with divisible. You can cut off a finger and have 'two parts of your body'. You cannot cut off a piece of your mind and have two parts of your mind. The problem with that is that you can cut off bits of your mind. A lobotomy, for example, will result in two parts of your mind that do not always work together and may be unaware of what the other is doing. We also may loose part of our mind (the ability to speak, to comprehend, to taste or see, to plan, to remember, etc.), which means that these parts of the mind are indeed divisible. However, this objection was not made apparent to Descartes, so he remained satisfied with the philosophy of dualism. But now you know better!

Rene Descartes on the mind/body problem, or philosophical dualism. In more ways than one, Descartes' distinction between the human mind and the human body is the starting point for modern philosophy with its insistence on the important of epistemological problems -- problems that in Kant's day focused on the transcendental possibility of knowing, and in more contemporary philosophies tend to focus on the physical possibility of mentality (i.e., philosophy of mind).

Descartes conceived of his own thinking as distinct from the physical activity of his body -- the I, according to Descartes, is a thinking thing, a thing that thinks. The thinking I is connected to the body only through the pineal gland a small, suspended, organ in the middle of the brain that 'can move in as many ways as the body can move', and represents for the body, the thought of the mind. On Descartes' view, mind and body are separate substances. His metaphysics is, therefore, dualistic, insofar as substances are (according to the thought of the day) self-complete and self-caused things.

"It is certain that I, that is to say my mind, by which I am what I am, is entirely and truly distinct from my body, and may exist without it" (Meditations, 156).

"We perceive bodies only by the understanding which is in us, and not by the imagination, or the senses... we do not perceive them through seeing them or touching them, but only because we conceive them in thought" (Meditations, 112).