I first read of the ‘paradox’ in one of Raymond Smullyan’s excellent books. My favourite formulation goes like this: A philosophy professor tells the class the

I first read of the ‘paradox’ in one of Raymond Smullyan’s excellent books. My favourite formulation goes like this: A philosophy professor tells the class there will be a surprise exam this week. He and the class discuss the exam, and they ‘prove’ that there can be no surprise exam. When everyone has accepted the proof, he announces that there will be an exam on the spot! The exam has only one question: Prove that the exam is indeed a surprise, invalidating the proof that there can be no surprise exam. This formulation suggests that the exam is a surprise if the students convince themselves that there can be no surprise exam, which is an entertaining trail to follow. Smullyan also tells an anecdote from his childhood. He and his brother liked to ‘fool’ each other on April Fool’s, and one year his brother announced that the coming April Fool’s Day, he would fool Raymond as he had never been fooled before. Well, the day came and Raymond steeled himself for the prank. He waited, expecting a prank or lie with everything they did and every cinversation they had, but no prank, no lies, no fooling. As he lay in bed that night, Raymond realized that his brother had, in fact, fooled him. Or had he? How could not fooling someone constitute fooling him? According to Raymond, puzzling over the nature of ‘fooling’ was part of his lifelong journey into philosophy and logic.