Where this idea comes from
Knights and knaves
The American logician and magician Raymond Smullyan filled an island with knights, who always tell the truth, and knaves, who always lie, in his puzzle book What Is the Name of This Book? (1978).
One thing to hold: Two islanders, one sentence each, and no way to tell them apart by looking.
Why it matters: You solve it by supposing: if this speaker were a knight, would everything still fit? If not, they must be a knave.