Full Idea
It is impossible for anything to be like an absolute idea, because a third idea will appear to make them alike, and if that is like anything, it will lead to another idea, and so on.
Gist of Idea
Nothing can be like an absolute idea, because a third idea intervenes to make them alike (leading to a regress)
Source
Plato (Parmenides [c.366 BCE], 133a)
Book Reference
Plato: 'Plato IV (Cratylus,Parmenides,Hippias Maj, Min)', ed/tr. Fowler,H.N. [Harvard Loeb 1926], p.221