On the Real Distinction Between Mind and Body
If
the mind and the body were identical, then it follows trivially that, when one
perishes, so does the other. Given that
Descartes dedicated his Meditations on First Philosophy to the theology
faculty at the
The bulk of the work of Descartes’ argument is performed by the principle of the necessity of identity: if two things are identical, they are necessarily identical. Applying this principle to mind and body, we see that if the two are identical, they could not possibly be separated from each other. In order to prove that they are not identical, then, Descartes presents an argument that the mind and body can be separated from each other (at least by God). This argument includes three additional premises, each of which gets some support in the previous five meditations. First, we have that the mind is essentially a thinking thing that can be understood as “merely a thinking thing and not an extended thing.”[1] Second, we have that the (human) body can be understood as “merely an extended thing and not a thinking thing.” Finally, Descartes claims, “all the things that I clearly and distinctly understand can be made by God such as I understand them.”
From the first premise, we have that a mind can be understood as a thinking thing having no extension, and from the second we have that a body cannot be understood but as a thing having some extension. Thus, we can clearly and distinctly understand a mind existing separate from any body. (A brief consideration of the nearest human corpse should convince anyone that the body can exist without a mind, which is a related but less important consequence of the first two premises.) Descartes combines the necessity of identity (in its equivalent contrapositive formulation) and the third premise when he states, “my ability clearly and distinctly to understand one thing without another suffices to make me certain that the one thing is different from the other, since they can be separated from each other, at least by God.” Since the first and second premises demonstrate that we can clearly and distinctly understand a mind existing without a body, we are thus led to the conclusion that mind and body are indeed distinct things that can exist separately.
On this reading, which seems to be the most natural way to understand how Descartes intends his argument to work, the logic is valid. The principle of the necessity of identity, the three premises Descartes states explicitly, and the negation of his conclusion that mind and body are distinct, form an inconsistent set of propositions. What remains to be shown by someone arguing in favor of Descartes’ conclusion is that all the premises of the argument for the real distinction between mind and body turn out to be true. The necessity of identity is the one premise Descartes does not state explicitly or give additional support for. Perhaps it appeared so self-evident to him that it survived the doubt of the First Meditation completely unnoticed. For the time being, we will allow it to survive our own doubt, as well, and first address the premises for which Descartes does argue separately in the Meditations.
First, there is the premise that the mind is a thinking thing that can be coherently understood merely as thinking and not as extended. The first instance of this claim can be found in the Second Meditation, when Descartes says, “I am therefore precisely nothing but a thinking thing; that is, a mind, or intellect, or understanding, or reason.”[2] It is impossible to think or utter the phrase, “I am, I exist,” without that phrase being true. I can only be certain of my own (mind’s) existence when I am presently thinking. So if I am thinking, my mind must exist, and if I’m sure my mind exists, I must be thinking. This gets us to the point where thinking occurs if and only if the mind exists, and it seems to follow from there that the mind is essentially a thinking thing and that it can be understood as merely a thinking thing. But it is actually not immediately clear that no other quality is as intimately connected with minds and thinking as the two are with each other. For instance, perhaps it is somehow as essential to the existence of a mind for it to be extended in space as it is for it to be thinking, except that we are not as readily aware of this fact.
More straightforward is Descartes’ claim that the body is essentially an extended thing, clearly and distinctly understandable without any other qualities. It is built into our notion of “body” that it has some extension in space. Even if, as might be possible in the case of the mind, there is some as yet unknown characteristic essential to a thing’s being a body, it seems uncontroversial that extension in space, at the very least, is a necessary characteristic. To claim that something could be a body and at the same time have no spatial extension is certainly to misunderstand what is meant by the terms involved. So if it did turn out to be the case that the mind can be clearly and distinctly understood as existing without spatial extension, it follows that the mind can be clearly and distinctly understood as existing apart from any body.
The premise that whatever I can clearly and distinctly understand can be made by God as I understand, if true, takes us from here to the conclusion that it actually is possible for the mind to exist apart from any body. Without going into any appreciable detail whatsoever, let us grant Descartes that the “truth rule” for things we clearly and distinctly understand allows at least for psychologically stable beliefs. That is, let us grant that the belief, “whatever I clearly and distinctly understood at some time is true,” is justified when clear and distinct understanding is actually occurring, and is at least unshakeable by doubt the rest of the time. Descartes argues from the principle of clear and distinct understanding to the conclusion that God is not a deceiver, and if that is granted, then God would not deceive us into clearly understanding anything that could not actually be brought about, at least by God himself.
By assuming this “justification” for the third premise, we are also in better position to address the necessity of identity. Rather than uncritically allowing this principle to survive his doubt in the First Meditation, Descartes may have simply perceived its truth too clearly and distinctly every time he considered it for the principle to be effectively shaken by the method of doubt. After all, it does seem to me to be pretty clear, every time I think about it, that if it is at all possible for one thing to exist without the other, then the two things cannot be identical. This, of course, is logically equivalent to the statement that identical things are necessarily identical.
However, even reading Descartes fairly generously and granting him that God exists and is not a deceiver and that whatever is clearly and disctinctly understood can be made by God such as it is understood, there remains a problem with the very first premise. Descartes asserts rightly that in thinking at all, we are made certain that the mind exists, whereas it remains unclear whether there is a body to go along with it. But this means only that we can clearly and distinctly understand the mind existing without simultaneously understanding it as having a body. It does not mean that we can clearly and distinctly understand the mind as existing without having a body. More precisely, understanding the mind as a thinking thing without understanding it as an extended thing is different from understanding the mind as a thinking thing and understanding it as an unextended thing. While the first understanding is clearly possible, it is not entirely clear that anything capable of causal interaction with extended bodies can be distinctly understood as having no extension itself. If it turns out that such a thing cannot be clearly and distinctly understood, then there is no way we could clearly and distinctly understand the mind as existing separate from the body. In that case, there is no reason to suppose that God can make them separate, and thus that they are really distinct from each other.