Full Idea
Form is a more plausible candidate for being nature than matter is because we speak of a thing as what it actually is at the time, rather than what it then is potentially.
Gist of Idea
Form, not matter, is a thing's nature, because it is actual, rather than potential
Source
Aristotle (Physics [c.337 BCE], 193b07)
Book Reference
Aristotle: 'Physics', ed/tr. Waterfield,Robin [OUP 1996], p.35
A Reaction
Note that matter remains potential, even when it is part of an actual thing. This seems to be the obvious point that a statue isn't potentially anything else, but its clay is potentially other objects. Does Aristotle think clay is thereby less real?