Full Idea
In Aristotle's theory a substantial form can count as a proper subject, since the generic matter of which the form is predicated is in fact a property of the form rather than the form's being a property of it.
Gist of Idea
A substance is a proper subject because the matter is a property of the form, not vice versa
Source
report of Aristotle (Metaphysics [c.324 BCE], ousia) by Mary Louise Gill - Aristotle on Substance Ch.5
Book Reference
Gill,Mary Louise: 'Aristotle on Substance: Paradox of Unity' [Princeton 1989], p.167
A Reaction
I'm not sure if I understand the idea of matter being the 'property' of a form, but 'matter' [hule] seems to be a particular way of thinking about stuff when it participates in an object, rather than just the amorphous stuff. Just 'predicated of'?