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Wellsprings
metaphysics

Non-Contradiction

ἀρχὴ τῆς ἀντιφάσεως

The same thing cannot both be and not be, in the same respect, at the same time.

The principle of non-contradiction, developed most rigorously by Aristotle, states that something cannot both be and not be in the same respect at the same time. This rule became the bedrock of classical logic and metaphysics. Aristotle treated it as so fundamental that he argued even skeptics rely on it whenever they make any claim—to deny the principle requires affirming it. The principle applies to properties, states, and relations: a thing cannot simultaneously be white and not white, or both exist and not exist in the same way.

The principle constrains what rational thought can entertain. If someone claims that Socrates is both wise and not wise in the same respect, they have violated the rule and made nonsense of language itself. This is why contradictory statements about the world cannot both be true. The principle eliminates logical incoherence as a possibility in reasoning about reality, making it impossible for contrary attributes to belong to the same subject simultaneously. Without it, rational discourse collapses into meaninglessness.

Where this idea shows up

36 Greek sources

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