Full Idea
Aristotle's argument is that if we understand the substance of a thing to be that which unifies it, and if we understand that a universal is predicated of many things, then we will see that a universal cannot be the substance of a thing.
Gist of Idea
Substance is unified and universals are diverse, so universals are not substance
Source
report of Aristotle (Metaphysics [c.324 BCE], 1038b1-15) by Charlotte Witt - Substance and Essence in Aristotle
Book Reference
Witt,Charlotte: 'Substance and Essence in Aristotle' [Cornell 1994], p.158
A Reaction
Presumably if universals are predicated of something, or something 'partakes' of the universal, then we want to know about the 'something', not about the universal. But do we end up with substances being 'bare particulars'?