Full Idea
If we allow Forms or numbers, they will not be the cause of anything, or, if that is too strong, they will at any rate not be the cause of any movement.
Gist of Idea
Pure Forms and numbers can't cause anything, and especially not movement
Source
Aristotle (Metaphysics [c.324 BCE], 1075b23)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.388
A Reaction
This is Benacerraf's famous observation (1973) that we can't accept a platonic account of numbers because, lacking causal powers, they are unknowable.