Full Idea
Plato's doctrine was that the Forms and mathematicals are two substances and that the third substance is that of perceptible bodies.
Gist of Idea
Plato's holds that there are three substances: Forms, mathematical entities, and perceptible bodies
Source
report of Plato (works [c.375 BCE]) by Aristotle - Metaphysics 1028b
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.171