A logical treatment of noise: Solving The Hardest Logic Puzzle Ever and its generalizations

Abstract

Raymond Smullyan came up with a puzzle that George Boolos called The Hardest Logic Puzzle Ever.[1] The puzzle has truthful, lying, and random gods who answer yes or no questions with words that we don't know the meaning of. The challenge is to figure out which type each god is. The puzzle has attracted some general attention -- for example, one popular presentation of the puzzle has been viewed 10 million times.[2] Various "top-down" solutions to the puzzle have been developed.[1,3] We present a systematic bottom-up approach to the puzzle and its generalization. We prove that an n gods puzzle is solvable if and only if the random gods are less than the non-random gods, for arbitrary cardinals. We develop a solution using 4.15 questions on average to the 5 gods variant with 2 random and 3 lying gods. We introduce an algorithm and an implementation for finding solutions to the generalized problem, together with upper bounds. Finally, we note that random gods act like noisy sources, which provides a connection to fault-tolerant computing.