  However many instances of each kind there might be, there is a dualism of two kinds of substance, according to Descartes: thinking things, or minds, and extended things, or bodies. This dualism generated two well-known problems, resolved by Descartes with only partial success. One of his most incisive critics, Elisabeth of Bohemia, wanted to know how in voluntary action the will, which is a property of the unextended mind, could have an effect on the body, given that, according to Descartes’s mechanistic physics, a material thing can be affected only by what is in contact with it. Descartes replied with a rather mysterious account of how the mind and body formed a unique kind of composite. Over the course of the letters of their correspondence that focus on this issue, Elisabeth presses Descartes for an explanation of how mind and body interact. Descartes’s responses do not satisfy Elisabeth, and rightly so. For, the mysterious composite that he suggests involves the concept of a primitive notion (Shapiro, 65/AT 3:665), a concept that is not present in any other of his writings. Descartes states that we have primitive notions for the body, “extension”, for the mind, “thought”, and for the mind/body composite, “union.” He explains that each of these notions is best understood by using a faculty that is particularly well-suited to grasp it. The primitive notion for the mind, he states, is best understood by the understanding alone; the primitive notion of the body is best understood by the understanding aided by the imagination; and the primitive notion of the composite is best understood by the senses (Shapiro, 69/AT 3:691). Armed with the distinction in methodology for knowing each of these notions, Descartes diagnoses Elisabeth’s difficulty in understanding his explanation in the following way: She is trying too hard to understand the union of her mind and body by her understanding alone. The remedy to this problem, he suggests, is for the Princess to stop meditating on the problem, and to instead focus on “life and ordinary conversations” in order to conceive the union (Shapiro, 70/AT 3:692). But, of course, this recommendation does not adequately address Elisabeth’s question. For, while focusing on “life” and “ordinary conversations” may well help to conceive that the mind and body interact, a point that Elisabeth does not call into question, it furnishes nothing towards an answer to the question of how such an interaction occurs. The discussion of this topic comes to an end with Elisabeth’s suggestion that she is better able to understand the how of interaction by thinking that part of the soul is extended (Shapiro, 72/AT 4:1). On this model, a necessary condition for motion, contact, is possible.