Full Idea
There must be knowledge of the one, or else not even the meaning of the words 'if the one does not exist' would be known.
Gist of Idea
We couldn't discuss the non-existence of the One without knowledge of it
Source
Plato (Parmenides [c.364 BCE], 160d)
Book Reference
Plato: 'Plato IV (Cratylus,Parmenides,Hippias Maj, Min)', ed/tr. Fowler,H.N. [Harvard Loeb 1926], p.313