The place of matter in the definition of the sensible substance evolves over the course of Metaphysics books Z and H : whereas chapters 10 and 11 of book Z claim that matter must be excluded from the definiens of the sensible substance, chapter 2 of book H claims that it must include it. The present article will show that this difference in doctrine comes from a refinement of the way in which the causal priority of form is expressed in (...) the definition. First, in book Z, it is expressed by a reduction of the terms of the definiens to « parts of the form » only, a reduction which is immediately considered superfluous and unsatisfactory. In book H then, it will be by the position it occupies as a predicate of the matter that is precisely meant to express this priority. In this sense, the two sets of texts do not constitute a theoretic volte-face from Aristotle, but the refinement of a single model of definition expressing the priority of form. (shrink)