Full Idea
It may seem that matter is substance, but this cannot be so, because what we think to be the central features of substance are separability and thisness. Then it seems more plausible to say that the form and the composite are substance than matter is.
Gist of Idea
Matter is not substance, because substance needs separability and thisness
Source
Aristotle (Metaphysics [c.324 BCE], 1029a27)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.175
A Reaction
This is an important basic point, because modern materialism takes matter (of some sort) to be basic, but Aristotle seems to take identity (and form and essence) to be basic, and matter to be merely at their service.