Full Idea
Aristotle thinks there are three main candidates for primary being: the particular, the universal and the essence, and Aristotle will defend the third of these.
Gist of Idea
The three main candidates for primary being are particular, universal and essence; essence is the answer
Source
report of Aristotle (Metaphysics [c.324 BCE], 1028a33-6) by Vassilis Politis - Aristotle and the Metaphysics 1.4
Book Reference
Politis,Vasilis: 'Aristotle and the Metaphysics' [Routledge 2004], p.12
A Reaction
If you really want to understand this idea, you must study this bit of the text carefully, and examine the translation of key terms like 'ousia'. Lawson-Tancred's translation gives a very different picture from Politis's commentary!