 Descartes, like the atomists, believed that matter operates in an entirely mechanical way. There is, therefore, no causal role for substantial form to play and, hence, no need for such forms. His two substances are each defined in terms of one property (extension for matter and thought for mind), hence there is no problem about the relation between substance and the properties in terms of which it is defined. As he does not have substances as individuals made of kinds of stuff, there is no conflict between individuals and stuffs.