The main aim of my work – “Matter and Explanation. On Aristotle's Metaphysics Book Η” – is to show the argumentative unity of Book Η (VIII), which has been usually regarded as a mere collection of appendices to the previous Book Ζ. In my thesis I take on the main suggestion provided by M. Burnyeat in “A Map of Metaphysics Ζ” (2001). According to Burnyeat, Η accomplishes the enquiry of Ζ by developing Ζ17's fresh start into the analysis of sensible substances. Starting from Ζ17, Aristotle regards the notion of substance in its explanatory role as “principle and cause” and, as a consequence, he searches for “the cause by reason of which a certain matter is some definite thing”. Burnyeat's suggestion has been so far followed in order to look at Η as at that place where this search is accomplished. Thus, Η would play a didactical-expository role. In my work I aim at showing how in Book Η Aristotle does not confine himself to a mere exposition of the previous outcomes. By contrast, he provides a deep revision of the status of matter's substancehood. Namely of that ontological subject whose organization must be explained. Such a revision concerns those criteria, which in Book Ζ have provided a deflationary reading of the notion of ὕλη. On the contrary, in Η matter is read as subject of physical changes and in its dispositional role within the biological wholes. Such a framework is accomplished in Η6, where Aristotle shows the explanatory primacy of his own hylomorphism over the Platonic Doctrine of Forms.