I also find that the senses show me that the soul moves the body, but they teach me nothing (no more than do the understanding and the imagination) of the way in which it does so. For this reason, I think that there are some properties of the soul, which are unknown to us, which could perhaps overturn what your Metaphysical Meditations persuaded me of by such good reasoning: the nonextendedness of the soul. This doubt seems to be founded on the rule that you give there, in speaking of the true and the false, that all error comes to us in forming judgments about that which we do not perceive well enough. Though extension is not necessary to thought, neither is it at all repugnant to it, and so it could be suited to some other function of the soul which is no less essential to it.
Elisabeth to Descartes, 1 July 1643 (tr. Shapiro)
When Elisabeth’s correspondence with Descartes is mentioned, it is often merely to credit her with being (among) the first to raise the interaction problem for substance dualism. But this radically understates what she’s doing here, and the depth of understanding of Descartes’s system she demonstrates. The brief quotation above is one of the most decisive refutations in the history of Western philosophy. Very probably, Descartes knows it and this is why he completely changes the subject in the next letter. As Leibniz observed, “Descartes had given up the game at this point” (“New System of Nature” (1695), tr. Ariew and Garber, p. 142).
The problem is this: the fundamental starting point of Descartes’s system is the claim that by the pure intellect we grasp the essence of body (extension) and the essence of mind (thought), and we can see that these two natures have nothing in common. Further, he claims, every feature of an entity must be a modification of its essence. Thus every feature of a body must be some particular manner of being extended, and every feature of a mind must be some particular manner of thinking. Further, Descartes is committed to the a priori intelligibility of causal relations. However, since extension and thought are utterly conceptually independent no a priori causal connection between any mode of extension and any mode of thought is possible. Thus if, as Descartes claims, we know by experience that the soul moves the body (i.e., causes the body to move), then mind, body, and causation are not thoroughly intelligible as Descartes supposes. Not only does this undermine Descartes’s argument for the real distinction of mind and body; it undermines most of his philosophical system. Game over, turn the lights out, it’s time to go home.
(Cross-posted at blog.kennypearce.net)