Full Idea
If a person denies that the idea of each thing is always the same, he will utterly destroy the power of carrying on discussion.
Gist of Idea
If you deny that each thing always stays the same, you destroy the possibility of discussion
Source
Plato (Parmenides [c.366 BCE], 135c)
Book Reference
Plato: 'Plato IV (Cratylus,Parmenides,Hippias Maj, Min)', ed/tr. Fowler,H.N. [Harvard Loeb 1926], p.229
A Reaction
Actually a concept can still be discussed even by people who declare slightly different understandings of the concept. God, Loch Ness monster, genius, atoms… I guess the surrounding language must be stable to achieve that.