Full Idea
A thing may be formally indivisible, something cognitively and scientifically indivisible. Hence what cause substances to be single things should be thought of as the primary unity.
Gist of Idea
The formal cause may be what unifies a substance
Source
Aristotle (Metaphysics [c.324 BCE], 1052a31)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.286
A Reaction
This is his fourth and final proposal for unity, and it is obviously his preferred theory, because it is the hylomorphic view, that the form or nature of the thing bestows the unity. It is sort of right, but a rather thin theory as it stands.