The notion of substance is meant to address the question, What are the metaphysically most fundamental entities? Substances have traditionally had the following features, although not all philosophers attribute all these features to substances:

1. 1.Substances exist in and of themselves and are neither reducible to nor dependent on more fundamental entities.

2. 2.They remain the same in a process of change; they are the substrata of change.

3. 3.They are genuine unities; they have per se unity, in the language of Descartes’ time.

4. 4.They retain their identity over time.

5. 5.There is an answer to the question what individuates substances at a time.

Descartes’ writing displays a clear position on the first two features; matters are complicated for the third, per se unity; and the last two features are not focal points of Descartes’ analysis of substance:he does not address identity over time or individuation for minds, and when he does so for bodies, it is not connected to his notion of substance. Before addressing the most controversial question about what Descartes counts as substances – in particular, whether he thought there is a multiplicity of corporeal substances and whether he thought human mind-body composites are substances – we first consider Descartes’ own most prominent characterizations of substance.

1.What Is It to Be a Substance?

The notion of substance as it came down to Descartes originates in Aristotle's division of beings into ten categories. These categories contain, on the one hand, the category of substance and, on the other hand, nine categories of accidents, to use the Scholastic term. This means that, intuitively speaking, it is the distinction between individual things and all manner of qualities, properties, states, and relations of things. This distinction between substance and accident is helpful to understand Descartes’ notion of substance. A substance is, in terms used by Descartes and his Scholastic predecessors, something that exists per se, in its own right. On the other hand, accidents (or qualities) exist per aliud or through another – that is, they exist by inhering in another thing, typically a substance. It is of the nature of a quality that it must so exist: a shape cannot exist by itself free-floating.