最終更新日:2022/12/24

Consider Peirce's law, ((P→Q)→P)→P). If Q is true, then P→Q is also true so the law reads If truth implies P then deduce P which certainly makes sense. If Q is false, then (P→Q)→P≡(P→⊥)→P≡¬P→P≡¬P→P and ¬P≡¬P→⊥≡¬¬P so the law reads ¬¬P→P, which is intuitionistically false but equivalent to the classical axiom ¬P∨P.

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