The connection between Aristotle’s theory of science and his mature metaphysics has been discussed in a number of different ways. In this article I explore in which way the close correlation between what Aristotle calls definition (horismos) and explanation (apodeixis) is elaborated in Metaphysics VII–VIII and how this elaboration contributes to a better understanding of Aristotle’s theory of science in the Posterior Analytics. The crucial claim I try to defend is that only if we adequately understand Aristotle’s solution of the problem of the unity of form-matter-composita (substances) in Metaphysics VII–VII can we see, according to Aristotle, what it is exactly what we get to know if we have successfully established scientific definitions by constructing scientific demonstrations. The main idea is that general form-matter-composita, being typical subjects of scientific definitions, do not consist of form F and matter M, but rather come down to a metaphysically necessary unity of form F and the general matter-form M. This necessity flows from a partial structural identity of forms F and M and grounds explanatory relations. It is in this way that Aristotle’s mature metaphysics completes his theory of science as it is presented in the Posterior Analysics.