The Spy Who Claimed to Be a Spy
Three people, Ada, Bo and Cy, are exactly one knight (always tells the truth), one knave (always lies) and one spy (may say anything, true or false). Each of them knows who is who. They say: - Ada: "I am the spy." - Bo: "I am the spy." - Cy: "Ada is the spy." Who is the knight? Answer with the name.
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Source: Original puzzle in the knights-and-knaves tradition popularised by Raymond Smullyan. Statement written for AxiomIQ.