Full Idea
Substance [ousia] is a principle [arché] and a kind of cause [aitia].
Gist of Idea
Substance is a principle and a kind of cause
Source
Michael V. Wedin (Aristotle's Theory of Substance [2000], 1041a09)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.227
A Reaction
The fact that substance is a cause is also the reason why substance is the ultimate explanation. It is here that I take the word 'power' to capture best what Aristotle has in mind.