Three islanders: a truth-teller, a liar, and one who can do either
You meet three islanders: one is a knight who always tells the truth, one a knave who always lies, and one a spy who can say either. Red says 'I am a knight.' Blue says 'Red is telling the truth.' Green says 'I am the spy.' Who is the knight, who is the knave, and who is the spy?
Reveal the answer
Red is the knight, Blue is the spy, and Green is the knave. Green's claim to be the spy can't hold up under either remaining role without contradiction, so Green must be lying, which makes Green the knave; testing the last two roles against Red and Blue's statements leaves exactly one consistent assignment, Red truthfully a knight, Blue a spy freely repeating a true remark. Raymond Smullyan introduced the spy as a devious third option on his classic knights-and-knaves format in his 1978 puzzle collection.
— Raymond Smullyan, What Is the Name of This Book? — 1978
Go deeper: get the book →