Full Idea
To everything that admits of a contrary there is one contrary and no more.
Gist of Idea
Only one thing can be contrary to something
Source
Plato (Protagoras [c.391 BCE], 332c)
Book Reference
Plato: 'Protagoras and Meno', ed/tr. Guthrie,W K C [Penguin 1956], p.65
A Reaction
The sort of thing for which a modern philosopher would demand a proof (and then reject when the proof couldn't be found), where a Greek is happy to assert it as self-evident. I can't think of a counterexample.