Full Idea
Aristotle wanted to argue (against Plato) that changing, material things, and not only universals that are true of them, have an essence.
Gist of Idea
Aristotle says changing, material things (and not just universals) have an essence
Source
report of Aristotle (Metaphysics [c.324 BCE], ess) by Vassilis Politis - Aristotle and the Metaphysics 7.5
Book Reference
Politis,Vasilis: 'Aristotle and the Metaphysics' [Routledge 2004], p.215
A Reaction
This is the huge idea which Aristotle contributes to our understanding of the world, and which I take to be one of the most important ideas in philosophy (though I accept that defending essences is a little precarious).