Most seventeenth-century thinkers regarded mind and body as distinct entities, though a few philosophers, such as Hobbes, did embrace versions of radical materialism. But dualism, as this chapter shows, inherently presents certain fundamental problems. We shall first discuss two distinct but closely related issues: (1) the nature of the ‘union’ between mind and body in a given human individual and (2) the question of whether mind and body can and do interact causally – and, if not, what may truly be said about the relation between a given state of mind (such as a pain or an intention) and a state of body (such as a pinprick or arm movement) normally taken to be its cause or effect. Descartes's position on these two problems is of particular interest in that he established the framework of argument – for both followers and opponents – throughout the seventeenth century. Then in Section II we shall turn to later dualists' responses. Finally, in Section III we shall address important questions for dualism connected with the new mechanist world-view. The most crucial from this perspective is perhaps the following: What is the relation between human volitions, conceived as irreducibly non-physical states, and the universal laws of motion which lie at the heart of the new science? If volitions are genuinely effective, must they result in unacceptable disruption of the uniform working of such laws?