 Descartes believed in only two kinds of substance: material body, which is defined by extension, and mental substance, which is defined by thought, which, in this context, is more or less equivalent to consciousness. Descartes (like Aristotle and unlike most of his contemporaries and immediate successors) was not an atomist. He did not believe in a void between bodies, so there is a sense in which there is just one material substance, numerically as well as specifically. For Descartes, therefore, material substance falls more naturally into the stuff category, rather than into the thing category. The situation is different for mental substance. The cogito shows that Descartes definitely believes that each person is a different individual mental substance.