Full Idea
One member of a pair of contraries is sufficient to discern both itself and its opposite.
Gist of Idea
From one thing alone we can infer its contrary
Source
Aristotle (De Anima [c.329 BCE], 411a02)
A Reaction
This obviously requires prior knowledge of what the opposite is. He says you can infer the crooked from the straight. You can hardly use light in isolation to infer dark [see DA 418b17]. What's the opposite of a pig?
Book Reference
Aristotle: 'De Anima (on the psuche)', ed/tr. Reeve, C.D.C. [Hackett 2017], p.18