Full Idea
Aristotle distinguishes three different answers to the question 'What is primary being?'; it is universals (which he thinks is Plato's answer); or it is the ultimate subject of predication (his answer in 'Categories'); or the essence (in 'Metaphysics').
Gist of Idea
Primary being is either universals, or the basis of predication, or essence
Source
report of Aristotle (Metaphysics [c.324 BCE], 1028b25-) by Vassilis Politis - Aristotle and the Metaphysics 4.4
Book Reference
Politis,Vasilis: 'Aristotle and the Metaphysics' [Routledge 2004], p.110
A Reaction
I note that Michael Wedin argues that 'Metaphysics' is a development of 'Categories' rather than a change of view. The middle view has been unpacked nicely in modern discussions. The claim of essences needs more clarification.