Full Idea
These bodies (Fire, Water and the like) change into one another (and are not immutable as Empedocles and other thinkers assert, since 'alteration' would then have been impossible).
Gist of Idea
The Four Elements must change into one another, or else alteration is impossible
Source
Aristotle (Coming-to-be and Passing-away (Gen/Corr) [c.335 BCE], 329b1)
Book Reference
Aristotle: 'The Basic Works of Aristotle', ed/tr. McKeon,Richard [Modern Library Classics 2001], p.509
A Reaction
This is why Aristotle proposes that matter [hule] underlies the four elements. Gill argues that by matter Aristotle means the elements.