In any pub with at least one drinker, there's one person whose choice tells you everyone's
Assume a pub has at least one person who might, at some point, order a drink. Logician Raymond Smullyan claimed there's always at least one patron such that, if that particular person is drinking, then everyone in the pub is drinking. It sounds absurd — how could one person's choice guarantee something about everyone? Is it actually true?
Reveal the answer
Yes, it's logically valid, a real theorem of formal logic, though it plays like a joke. If everyone in the pub is drinking, any person satisfies the condition trivially. If not everyone is drinking, pick anyone who isn't: the statement "if they're drinking, everyone's drinking" is vacuously true, since the premise is false. Either way, such a person exists. Smullyan called it the Drinking Principle in his 1978 puzzle collection.
— Raymond Smullyan, What Is the Name of This Book? — 1978
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