Full Idea
He criticises his 'Categories' view, because if primary being is simply the ultimate subject of predication the primary being is, in virtue of itself, something indeterminate; it would be a necessary but not a sufficient condition for primary being.
Gist of Idea
Primary being must be more than mere indeterminate ultimate subject of predication
Source
comment on Aristotle (Categories [c.331 BCE]) by Vassilis Politis - Aristotle and the Metaphysics 7.5
Book Reference
Politis,Vasilis: 'Aristotle and the Metaphysics' [Routledge 2004], p.207
A Reaction
Thus, Politis argues, primary being is essence in the later work. The words 'substance' and 'ousia' cause confusion here, and must be watched closely. Wedin argues that Aristotle merely develops his 'Categories' view, but most disagree.