Full Idea
It has been held that Aristotle needed the conception of prime matter only because he held that the transmutation of one element into another is impossible.
Gist of Idea
Aristotle may only have believed in prime matter because his elements were immutable
Source
report of Aristotle (Metaphysics [c.324 BCE], matter) by Peter Alexander - Ideas, Qualities and Corpuscles 01.2
Book Reference
Alexander,Peter: 'Ideas, Qualities and Corpuscles' [CUP 1985], p.46