The ‘real distinction’ argument has its roots in arguments developed in the Second Meditation concerning knowledge of the self as a thinking thing, and knowledge of the body as something “extended, flexible, movable.” Having used the Evil Genius hypothesis in the First Meditation to bring into doubt the existence of the body, Descartes argues in the Second Meditation that this doubt does not extend to his own existence (if he deceives Descartes, Descartes exists.) He then considers what attributes with certainty to himself at this stage of his reasoning. He concludes that even certain properties, like nutrition, must be excluded as part of the doubt of body. There is only one, he finds, that is not called into question on this basis: “I am therefore, strictly only a thinking thing.”

This seems to imply that Descartes has established his argument: that he is nothing but a thinking thing, and as such distinct from anything physical: “thought alone cannot be separated from me.” But Descartes does not wish to claim on the basis of the Second Meditation reasoning alone that he knows that he is only thought and there is nothing corporeal or physical about him. He is implicitly claiming to know, not merely that he thinks, but that thought pertains to his nature or essence: it “cannot be separated from me.” Also, he maintains that reasoning concerning the indubitability of his own existence has brought him to the conclusion that he is a “truly existing thing.”

The Second Meditation contains at least one other assertion that is important to the ‘real distinction’ argument: that Descartes has a clear and distinct idea of himself as a thinking thing. He begins to hint at this point immediately after the statements already cited. And at the end of the Second Meditation, after arguing that his best knowledge of a typical physical object—a piece of wax—is derived from reason rather than sense, he concludes that his existence is more distinct and evidence than that of the wax. The claims about distinct perception are important because of Descartes’s very consciously held position that only clear and distinct perceptions or conceptions will suffice as the basis for positive affirmations about the nature of a thing.

Between the Second and Sixth Meditations Descartes validates his distinct perceptions by setting forth “proofs” of the existence of an omnipotent and benevolent creator who would not permit Descartes to be deceived in what is most evident. Descartes is, then, so far from concluding rashly from what he can conceive to what is the case, that he finds it necessary to present God as a bridge from what he can distinctly conceive to what is the case.

We may now turn to the ‘real distinction’ argument itself. The Sixth Meditation begins with the observation that God is capable of bringing about or making the case whatever I am capable of clearly and distinctly perceiving: “…And I never judged that anything could not be brought about by him, except for the reason that it was impossible for me to perceive it distinctly.” The first application of this principle is to establish the possible existence of “physical things conceived as the object of mathematics”—since previous Meditations have held these to be distinctly conceivable.

The second application is the real distinction:“…I am a thinking thing. And although probably (or rather, as afterward I will say, certainly) I have a body, which is very closely conjoined to me, because nevertheless on the one hand I have a clear and distinct idea of myself, insofar as I am only a thinking thing, not extended, and on the other hand I have a distinct idea of body, in so far as it is only an extended thing, not thinking, it is certain that I am really distinct from my body, and can exist apart from it.”I will provide a provisional reading of this argument, which to me seems natural:

(1) If A can exist apart from B, and vice versa, A is really distinct from B, and B from A.

(2) Whatever I can clearly and distinctly understand can be brought about by God.

(3) If I clearly and distinctly understand A apart from B, and vice versa, then God can bring it about that A and B are apart.

(4) If God can bring about A and B apart, then A and B exist apart.

(5) I am clearly and distinctly able to understand A apart from B if there are attributes Y and X, where I understand Y to belong to A only and X to belong to B only.

(6) Where A is my mind and B is my body, thought extension satisfy the conditions and Y and X respectively.

(7) Hence I am really distinct from my body and can exist apart from it.

What, if anything, is wrong with this argument? Whatever may be the connection in Descartes’s mind between his inability to doubt his own existence while doubting the existence of body it is perhaps not sufficiently captured by his real distinction, we may say. But it appears that if the mind and body are separate, that one may indeed be more certain than the other, which Descartes take to be the mind. Another objection may be that Descartes shows that mind and body are possibly distinct—and would be distinct if God chose to separate them—not that they are distinct. But Descartes holds that two things are really distinct if it is possible that they exist in separation. Therefore distinctness does not entail separation. It seems the objections are difficult to come by, so the next thing to ask is how one recognizes clear and distinct perceptions.

The Sixth Meditation concludes Descartes’s argument by means of the senses for the existence of body. Sensory perceptions must either be created by the Meditator himself, by someone or something else, or by God. The Meditator can rule himself out since he is not aware of creating these perceptions, and they come upon him so forcefully and involuntarily that it would be inconceivable that he could be the creative force behind them. This is proof enough that sensory perceptions have some outside cause. He is naturally inclined to think his sensory perceptions are caused by things that resemble those perceptions. Since God is not a deceiver, he must not be fooling him in giving him this natural inclination. Therefore, he concludes, bodies must be something like what they seem to be.

The discussion of sensory perceptions as being “caused” by some outside source marks an important turning point. The mind is sharply distinguished from the world of bodies around it. The Meditator argues that mind and body have nothing in common, so they must be two totally distinct substances. We could point out that Clark Kent and Superman are very dissimilar and are yet the same thing, and so argue by analogy that mind and body might be two very different ways of looking at the same thing. However, even the primary attributes of mind and body are different. Body is essentially extended, whereas mind is non-extended and essentially thinking. Since the two are totally different, the Meditator concludes that he is only mind, and not body. This is a step beyond what is stated by the sum res cogitans in the Second Meditation, as there the Meditator asserts that he only knows that he is a thinking thing. Now he knows that he is only a thinking thing.

This sharp distinction between mind and body has profound implications. If sensory experience is in the mind and the bodies that cause our sensations are in the world, the question arises as to how the two can causally interact. What is the connection between mind and world? When the mind and the world are held as totally distinct, the mind is then conceived of as being trapped within the body, unable to know about the world except through a “causal interface” at the sensory surfaces.