  The more modern versions of dualism have their origin in Descartes’ Meditations, and in the debate that was consequent upon Descartes’ theory. Descartes was a substance dualist. He believed that there were two kinds of substance: matter, of which the essential property is that it is spatially extended; and mind, of which the essential property is that it thinks. Descartes’ conception of the relation between mind and body was quite different from that held in the Aristotelian tradition. For Aristotle, there is no exact science of matter. How matter behaves is essentially affected by the form that is in it. You cannot combine just any matter with any  form – you cannot make a knife out of butter, nor a human being out of paper – so the nature of the matter is a necessary condition for the nature of the substance. But the nature of the substance does not follow from the nature of its matter alone: there is no ‘bottom up’ account of substances. Matter is a determinable made determinate by form. This was how Aristotle thought that he was able to explain the connection of soul to body: a particular soul exists as the organizing principle in a particular parcel of matter.