 According to Descartes, minds and bodies are distinct kinds of thing, or, in the technical terminology of the day, distinct kinds of substance. Bodies, he held, are spatially extended substances, incapable of feeling or thought; minds, in contrast, are unextended, thinking, feeling substances: souls. (We use “soul” with no theological implications to designate minds considered in the Cartesian way as immaterial substances.) Despite recognizing these deep differences, Descartes accepted the common belief that mind and body causally interact: “Everyone feels that he is a single person with both body and thought so related by nature that the thought can move the body and feel the things which happen to it” (in Cottingham et al. 1991, p. 228). But if minds and bodies are so radically different, it is not easy to see how they could interact. Descartes was well aware of the difficulty. Princess Elisabeth of Bohemia puts it forcefully to him in a 1643 letter, pressing Descartes to tell her