In GC II 5, Aristotle proceeds through a long and complicated argument against the view that there is a single primary body, concluding that a single simple body cannot function as the matter or origin of the other simple bodies. In doing so, he responds to a possible objection to his own account, defended in GC II 4, and confirms that each of the simple bodies is an origin for each of the others. This essay brings attention to the role of contrarieties in Aristotle’s refutation of theories that maintain a single primary body, either as the matter or the material origin of other simple bodies: given the role of contrarieties in explaining simple bodies, Aristotle finds that a single primary body is incompatible with the existence of change. By highlighting the role of contrarieties in explaining change, Aristotle leaves room for the primary contraries to function as a kind of matter, although the details of his account are not explored in GC II 5.