Full Idea
The explanation [aition - cause] that is the object of enquiry is that by virtue of which the matter is in the state that it is in. And this cause [explanation] is the form, and the form the substance [ousia].
Gist of Idea
The explanation is what gives matter its state, which is the form, which is the substance
Source
Aristotle (Metaphysics [c.324 BCE], 1041b08)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.228
A Reaction
A key sentence, I think, for understanding Aristotle's whole enterprise. The explanation is the essence; the essence is what explains.