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Wellsprings
metaphysics

The Excluded Middle

νόμος τοῦ ἀποκλειομένου τρίτου

Between true and false there is no third option — every statement falls on one side.

Aristotle formulated the law of excluded middle as a fundamental principle of logic: any proposition about the world must be either true or false, with no middle ground possible. This principle underlies his entire logical system and appears alongside the law of non-contradiction as essential to rational thought. Where non-contradiction says something cannot be both A and not-A, the excluded middle asserts that everything must be either A or not-A—there is no third option. The law ensures that contradictory statements about reality cannot both fail to be true.

The principle becomes practical in argument and reasoning. When Socrates establishes that a human must be either sick or not sick, with no third condition available, he uses excluded middle to force a choice and eliminate evasion. This matters for ethics and knowledge: if you must be either wise or unwise with nothing between, you cannot hide in ambiguity about your own understanding. The law makes dialectical questioning possible—it closes off retreat into vagueness and demands that contradictory claims be resolved.

Where this idea shows up

18 Greek sources

Where to read it