Named after the logician and philosopher Charles Sanders Peirce.
- (logic) The classically valid but intuitionistically non-valid formula of propositional calculus, which can be used as an substitute for the law of excluded middle in implicational propositional calculus.
- Consider Peirce's law, . If Q is true, then is also true so the law reads "If truth implies P then deduce P" which certainly makes sense. If Q is false, then so the law reads , which is intuitionistically false but equivalent to the classical axiom .