Law of excluded middle

The law of excluded middle states that for any proposition, either it or its contradictory obtains; for any proposition P, either P or not-P.

See law of non-contradiction for a summary of the differences between the law of non-contradiction, the law of the excluded middle, and the law of bivalence.

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If we remove the law of excluded middle from a formal logical system, the result will be a system called 'intuitionistic logic', which is the logic of mathematical intuitionism.