This study is concerned with a crucial passage in Metaphysics Z.11. After having established that only the formal parts of an object are stated in its definition and thus constitute its essence, Aristotle warns us against the process of separating the formal from the material parts. In doing so, he rejects the comparison proposed by Socrates the Younger. Mathematicals cannot be equated to natural objects because some material parts must be included in accounting for the latter but not in accounting (...) for the former. The goal of this article is to understand to what extent matter is essential to an object by examining the content of Aristotle’s criticism. My reconstruction shows that Aristotle is still committed to a formalist view. Socrates’ comparison is rejected because it removes matter not from the definitions of the subjects of metaphysics, but from the definitions of their attributes. (shrink)