The current debate over Aristotle's commitment to prime matter is centered on diachronic considerations found in his theory of substantial change. I argue that an appeal to this theory is not required in order to establish his commitment to the existence of prime matter. By drawing on Physics II.1's conception of what it is for an element to have a nature - that is, to have an inner source of movement and rest - I introduce a synchronic justification for the (...) existence of prime matter. By trading on the relationship between the thing that has a source of change and the source it has , I show that something that has a source in itself cannot be identical with its source, and that a type of matter that has no nature of its own (a kind of prime matter) is required to block this identification at the level of the elements. (shrink)