Aristotle's "Metaphysics H6" is a very disputed text. Though it is clearly devoted to the problem of the unity of definition, commentators diverge as to what the "definiendum" is supposed to be: is it "eidos", as Book Z seems to imply or rather "ousia" as a compound of matter and "eidos"?. In this paper, I propose to examine this issue by offering a new translation and a careful reading of the text which aims at reconstructing its different argumentative steps. I show how the different problems identified by commentators are interwoven and in what sense their solution might be considered as literally identical, provided that Aristotle's leading concepts are interpreted as "concepts of reflection". This interpretation also makes it possible to understand the difficult concept of "intelligible matter", of whose mentions I propose a general assessment. Finally, it sheds light on the argumentative sequence formed by Books Z, H and O of the "Metaphysics", and more precisely by their conclusive chapters