Why did Descartes hold to a dualism of the mental and the physical? We shall look at two of his arguments. A helpful way of understanding these arguments is by looking first at a law formulated after Descartes time by the German philosopher Leibniz (1646-1716). It is known by the name “the identity of indiscernibles” and states that for any x and y, if x and y have all the same properties, then x is identical to y. This implies a test for non-identity or difference: if we can find one thing true of x that is not true of y or vice versa, then x is not identical to y. Applied to the philosophy of mind this means that if we can find one thing true of mind that is not true of body then they are not the same. It should be noted though that this way of arguing will only support substance dualism if in the background we assume an understanding of the world as made up of substances with their properties.

The first argument we shall look at is the argument from doubt. Descartes sceptical arguments lead him to global scepticism where nothing seems to have any certainty at all. Yet at this point he finds that he cannot doubt that he is thinking and so existing. So at this point Descartes doubts the existence of his body; however he cannot doubt that he exists as a thinking thing. Therefore, by Leibniz’s law, this thinking thing must be distinct from the body.

Here is how Descartes argues:

Seeing that I could pretend that I had no body and that there was no world nor any place where I was, but that I could not pretend, on that account, that I did not exist … on the contrary, from the very fact that I thought about doubting the truth of other things, it followed very evidently and certainly that I existed … From this I knew that I was a substance the whole essence or nature of which was merely to think, and which, in order to exist, needed no place and depended on no material things. Thus this ‘I’ … is entirely distinct from the body”

Antoine Arnauld (1612-1694), a renowned theologian and philosopher, responded to many of Descartes arguments at his request. To this argument he formulated a parallel argument which he believed showed the flaw in Descartes reasoning. Arnauld’s argument concerns the properties of a right-angled triangle and proceeds by saying that while I may not be able to doubt certain essential features of the triangle, such that it is three-sided, I may well doubt (especially if not practiced in geometry) that the triangle possesses the Pythagorean property (that the square of the hypotenuse is equal to the sum of the squares on the other two sides). If Descartes arguments worked then in this analogous argument the result would be that the triangle does not possess the Pythagorean property, a result which is absurd. The problem with Descartes reasoning would therefore seem to be that what one knows with certainty or what one doubts reflects the intellectual condition of the thinker and therefore is not sure guide to the nature of the object being thought about.

Descartes reply was that this objection comes too early in the argument. The argument from doubt only suggests that we seem to be only a thinking thing. Later Descartes argues that we are, objectively, only a thinking thing and then gives his argument from clear and distinct perception:

“The fact that I can clearly and distinctly understand one thing apart from another is enough to make me certain that the two things are distinct, since they are capable of being separated at least by God … This, simple by knowing that I exist and seeing at the same time that absolutely nothing else belongs to my nature or essence except that I am a thinking thing, I can infer correctly that my essence consists solely in the fact that I am a thinking thing … accordingly, it is certain I am really distinct from my body, and can exist without it.”

Many have objected that the move from “I only know that I am a thinking thing” to “I know that I am only a thinking thing” is not fully argued by Descartes. However Descartes also questions whether Arnauld’s argument is really parallel to this own. Descartes’ argument is based upon the claim that one cannot doubt that one thinks, because to doubt is to think. Surely, though, it can be doubted that the figure in front of me is a right-angled triangle? Moreover, for Descartes, neither the triangle nor the Pythagorean property can be understood as substances and so cannot be thought to exist on their own.

Related LinksPhilosophy of mind, Distinctive features of the mind, Cartesian dualism