Full Idea
On our theory two is part of three….so it will not be possible for a number to be a Form, on pain of one Form's being present in another and all Forms turning out to be parts of some one.
Gist of Idea
If two is part of three then numbers aren't Forms, because they would all be intermingled
Source
Aristotle (Metaphysics [c.324 BCE], 1082b29)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.414