Knights and Knaves is a type of logic puzzle where some characters can only answer questions truthfully, and others only falsely. The name was coined by Raymond Smullyan in his 1978 work What Is the Name of This Book?[1]
The puzzles are set on a fictional island where all inhabitants are either knights, who always tell the truth, or knaves, who always lie. The puzzles involve a visitor to the island who meets small groups of inhabitants. Usually the aim is for the visitor to deduce the inhabitants' type from their statements, but some puzzles of this type ask for other facts to be deduced. The puzzle may also be to determine a yes–no question which the visitor can ask in order to discover a particular piece of information.
One of Smullyan's examples of this type of puzzle involves three inhabitants referred to as A, B and C. The visitor asks A what type he is, but does not hear A's answer. B then says "A said that he is a knave" and C says "Don't believe B; he is lying!"[2] To solve the puzzle, note that no inhabitant can say that he is a knave. Therefore, B's statement must be untrue, so he is a knave, making C's statement true, so he is a knight. Since A's answer invariably would be "I'm a knight", it is not possible to determine whether A is a knight or knave from the information provided.
Maurice Kraitchik presents the same puzzle in the 1953 book Mathematical Recreations, where two groups on a remote island – the Arbus and the Bosnins – either lie or tell the truth, and respond to the same question as above.[3]
In some variations, inhabitants may also be alternators, who alternate between lying and telling the truth, or normals, who can say whatever they want.[2] A further complication is that the inhabitants may answer yes–no questions in their own language, and the visitor knows that "bal" and "da" mean "yes" and "no" but does not know which is which. These types of puzzles were a major inspiration for what has become known as "the hardest logic puzzle ever".