A judge promises a surprise execution — and logic can't rule it out
A judge tells a prisoner he'll be hanged at noon on one weekday next week, and that he won't know which day until the executioner knocks. The prisoner reasons it can't be Friday (he'd know by Thursday night), which rules out Thursday too, and so on back to Monday — concluding the hanging can't happen at all, as a surprise or otherwise. Then, on Wednesday, the executioner knocks.
Reveal the answer
The prisoner is genuinely surprised, despite his airtight-seeming logic — and philosophers still disagree about exactly where the argument breaks down. Martin Gardner popularised the puzzle in his Mathematical Games column in Scientific American in March 1963; it remains one of the few paradoxes in logic with no agreed resolution.
— Martin Gardner, The Unexpected Hanging and Other Mathematical Diversions — 1969 (column originally published 1963)
Go deeper: get the book →