The Hardest Logic Puzzle Ever

The Hardest Logic Puzzle Ever came up recently in a Metafilter post, but it linked to a site that gave a very poor explanation of the puzzle and no credit to the original author, Raymond Smullyan (creator of the logic puzzles involving people who always tell the truth and people who always lie).

George Boolos wrote up a nice description of the Hardest Logic Puzzle Ever, including clarifications and breaking the puzzle down into more manageable sub-puzzles. The puzzle goes a step beyond the liars and truthtellers by introducing a third entity: someone who just gives random responses. And they all speak in some completely foreign language. If you solve the whole thing, you can claim that you solved the Hardest Logic Puzzle Ever! It is a bit of a project and benefits from contemplation. It's one of my more memorable puzzle experiences.

George Boolos also once gave a short talk on Gödel's second incompleteness theorem, using only one-syllable words. The text is available here in PDF.

And Smullyan does more than just make puzzles! He is a professor of philosophy at Indiana University and has done important work on extending the applicability of Gödel's incompleteness theorems.