Full Idea
Socrates: Maybe each of these forms is a thought and properly occurs only in minds. Then each of them might be one. …Parm: But won't this thing that is thought to be one, being the same over all the instances, be a form?
Gist of Idea
Maybe thoughts are just thoughts in minds - but how then do they cover many instances?
Source
Plato (Parmenides [c.366 BCE], 132b-c)
A Reaction
This seems to imply that a word or concept can only operate as a universal (a one-over-many) if it is an ideal form. They don't seem to consider options like resemblance nominalism. Or simply that 'horse' covers many horses because it is vague.
Book Reference
Plato: 'Parmenides', ed/tr. Gill,M.L./Ryan,P. [Hackett 1996], p.133