It is commonly assumed that Aristotle thinks that his claim that being exhibits a category-based pros hen structure, which he introduces to obviate the problem of categorial heterogeneity, is sufficient to defend the possibility of a science of being qua being. We, on the contrary, argue that Aristotle thinks that the pros hen structure is necessary only, but not sufficient, for this task. The central thesis of our paper is that Aristotle, in what follows 1003b19, raises a second problem for (...) the possibility of the science of being qua being; and that he does not think that the resolution of the first, the category-based problem, is either necessary or sufficient for resolving this problem. This is the problem: how can a plurality of apparently primary kinds and their opposites (they include to hen, to on, to auto, to homoion, to heteron and to anhomoion) be the subject-matter of the science of being qua being? It has been argued that these kinds are per se attributes of ousia and that, therefore, this problem is not different from the first problem. This, we argue, is mistaken; for nowhere in Gamma 2 does Aristotle claim that unity is a per se attribute of ousia. Rather, he says that identity, similarity, etc. are per se attributes of being qua being and unity qua unity. Aristotle’s resolution of the second problem, we argue, is that most of these kinds are reducible to a single compound principle: being-and-unity. Being and unity, moreover, are themselves related to each other as primary ousia and consequent ousia; but, we argue, Aristotle leaves it open, in Gamma 2, which of the two is primary, and which is consequent ousia. (shrink)