Full Idea
In general those things that do not admit of division [diairesis] are one insofar as they do not admit of it.
Gist of Idea
Things are one to the extent that they are indivisible
Source
Aristotle (Metaphysics [c.324 BCE], 1016b03)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.123
A Reaction
Aristotle gives a man, an animal and a magnitude as examples. The interesting thing here is that being 'one' seems to come in degrees, where most metaphysicians long for oneness to be an absolute.