The defense of this claim will be the central task of this paper. I shall begin with an exposition of the account Descartes gives of mind-body interaction in the letters Descartes wrote to Elisabeth in May and June of 1643, letters that form the first line of defense for Descartes' interactionism among those commentators who are committed to defending Descartes” position. After a short digression on a curious analogy Descartes makes between his position and the Scholastic account of heaviness and free fall, I shall examine Descartes' answer to Elisabeth in some detail, and argue that it is inconsistent with the foundations Descartes gives to his theory of motion. Finally, I shall attempt to sketch out an answer that Descartes could have given to Elisabeth in 1643, an answer that seems both philosophically interesting, and consistent with the rest of his writings. Before entering into the argument proper, though, I would like to make a few prefatory remarks concerning the issues I intend to take up, and the issues I don't. The issue that I intend to focus on is that of the intelligibility of mind-body interaction. The issue is, admittedly, a fuzzy one, as fuzzy as the notion of intelligibility itself. But historically speaking, it is an important one, as the reaction of Descartes' contemporaries and successors shows. To make the question a bit more precise, I shall construe it, as Descartes and his contemporaries often seemed to do, as the problem of whether the notion of mind-body interaction is somehow intelligible on its own terms, or whether its intelligibility requires an explication, analogy, or analysis in terms of some other distinct variety of causal interaction, itself more basic, or, at least, better understood. To be more precise still, given the prominence of the notion of impact in the then modish mechanistic world view, the question of the intelligibility of mind-body interaction quickly becomes a question of whether mind-body interaction can be understood without somehow relating it to the way in which bodies cause changes in one another through impact." The question of intelligibility should be distinguished from the closely related question of whether or not the mind and body do, as a matter of fact, actually interact with one another. Though Descartes and his correspondents and critics often link the two questions for obvious reasons, they are really somewhat independent. One can hold that despite the intelligibility of mind-body interaction, minds and bodies do not, as a matter of fact interact with one another. Philosophically, some reason must be given over and above the bare intelligibility of interactionism for adopting that position. Descartes does have an answer to this question, and an interesting one: it is experience, he claims,“the surest and plainest everyday experience,” as he writes to Arnauld, that convinces us of the truth of interactionism. But as important as this question is, it will not interest me here. My concern will be with bare intelligibility.