Two boxes, two labels, one true clue to find the treasure
A suitor must choose between a gold casket and a silver casket; one contains the treasure. The gold casket reads: 'The treasure is in this casket.' The silver casket reads: 'Exactly one of these two statements is true.' Which casket holds the treasure?
Reveal the answer
The silver casket. If the treasure were in gold, gold's statement would be true — but checking both possible truth values for silver's statement in that case leads to a contradiction either way. The only consistent assignment is: gold's statement is false (the treasure isn't in gold) and silver's is true (exactly one statement — silver's own — is true). So the treasure is in silver. Raymond Smullyan built dozens of these self-referential casket puzzles, inspired by Portia's suitor test in The Merchant of Venice, for his 1982 book The Lady or the Tiger?
— Raymond Smullyan, The Lady or the Tiger? And Other Logic Puzzles — 1982
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