Full Idea
There need be no forms (one item apart from the many) for demonstrations. But there must be universals, where one thing holds of the many. Without universals there are no middle terms, and so no demonstrations.
Gist of Idea
Separate Forms aren't needed for logic, but universals (one holding of many) are essential
Source
Aristotle (Posterior Analytics [c.327 BCE], 77a05)
Book Reference
Aristotle: 'Posterior Analytics (2nd ed)', ed/tr. Barnes,Jonathan [OUP 1993], p.16