Descartes insists that there is a mind/soul that is completely separate from the body. He considers the soul to be immaterial and the body to be physical. This is known as Cartesian Dualism. Descartes regards the soul as being non-spatial and being private whereas the body is public and spread out in space and time. He believes that the soul influences the body and vice versa via the pineal gland in the brain. He thinks that every sense we ‘feel’ is transmitted through our pineal gland to our soul, which then makes a decision about it and sends instructions back. Descartes knows the soul or something like it exists (cogito ergo sum) yet he does not know that the body exists yet. Descartes’ considers himself to be just a thinking thing, “But what then am I? A thinking thing. And what is that? A thing which doubts, understands, affirms, denies, wills, does not will, and which also imagines and senses.”
Descartes’ first argument for proving that the soul is distinct from the body goes as follows: it is possible to doubt your body’s existence but you cannot doubt that you exist and therefore your soul is distinct from your body. This argument seems fine at first but is it possible that Descartes has confused the fact that he can imagine himself without a body with the real possibility of him not having a body? Arnauld raised an objection over this, claiming Descartes’ argument from doubt to be a fallacy. Arnauld said that he could not doubt that a right-angled triangle is right-angled but he could doubt that a right-angled triangle does not obey Pythagoras’ Theorem. Therefore he could claim, using Descartes’ logic, that the Pythagorean property is not an essential part of a right-angled triangle. Descartes would have understood this argument in particular, as he was a mathematician as well as a philosopher.
However, Descartes offers a few more arguments for Dualism including the argument from clear and distinct perception. This line of reasoning claims that he has a clear and distinct perception of himself as only a thinking thing and so his essence cannot contain non-thinking extended (physical) things. Therefore his soul must be separate from his body and so can exist without it. Unfortunately, Descartes show us no proof that the body is not in fact a part of our essence. He must claim to understand his entire essence but he cannot know this.
Moreover, Descartes presents a third argument: the argument from indivisibility. He says that the mind is indivisible but the body is indivisible. Therefore, the mind must be distinct from the body. An objection to this is that the mind could be considered to be a different aspect of the whole person (body and mind).
In conclusion, Desacrtes presents several arguments for is entire essence being a thinking thing but none of them are completely satisfying. There are valid objections for all of them and Descartes struggles to explain how, exactly, the mind links with the body, as the pineal gland is obviously not the missing link.
Rene Descartes - Meditations and Other Metaphysical Writings (2000)
