The thesis examines the last aporia in Metaphysics (Met. B), which asks whether the principles are particular or universal. On the basis of Met. A and B, chapter 2 argues that the principles are primary in being and in knowledge. Moreover, I show that several aporiai are closely interconnected and favour a solution of universal substantial principles. My main argument (in chapter 2) for this claim rests on the apparent ambiguity of the term 'numnerical oneness' as between particular-style oneness and oneness of type. In chapter 3 I discuss the notions of separation and priority in Aristotle's Met. Aristotle rejects the Platonist view that Fonns can exist independently of all their sensible instances but not conversely. I argue that his account of priority is neutral between asymmetric independence in existence and in ''being what it is'. The latter type of independence is the ontological correlate of separation and priority in account. I show that principles or substantial forms satisfy this ontological criterion but not the existential one: they are what they are independently ofall their composite instances being what they are but not conversely. In chapter 41 offer a detailed commentary on Met. M.IO. I show how my discussion ofMet. B and of the notions of separation and priority is directly relevant in the aporetic part of the chapter (l086bI4-1087a4). In setting out Aristotle's metaphysical solution (I087a4-1O), I argue that the principles possess numerical oneness of type (a notion introduced in chapter 2): each principle is inexhaustibly repeatable but common to all particulars whose principle it is. Hence, the principles are universal but do not exist independently of particulars. Rather, they satisfy separation and priority in 'being what it is' (a notion introduced in chapter 3). In discussing I087alO-25 I argue that the epistemological aspect of the aporia rests on a conflation of two claims: (a) The form and the principles of knowledge are universal, while its objects can be universal or particular. (b) The form and the objects ofknowledge are universal. I show that claim (a) resolves M.IO's epistemological puzzles. Universal knowledge and principles can be of not only universal but also particular objects. For this reason, universal knowledge is characterised as potential: it can lead to grasp of particular objects.