Full Idea
On account of the difficulty [about unity] some philosophers have espoused participation, though this plunges them into difficulties about what the cause of the participation is, or indeed what participating is anyway.
Gist of Idea
If partaking explains unity, what causes participating, and what is participating?
Source
Aristotle (Metaphysics [c.324 BCE], 1045b07)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.250
A Reaction
The target here is Plato, and I agree with the criticism. Exactly the same problems face those who talk of an object 'instantiating' a property. I have no idea what such a relationship could be.