From Prof. Mark Liberman (Language Log), attributing this to Columbia philosophy professor Sidney Morgenbesser:
Morgenbesser was leaving a subway station in New York City and put his pipe in his mouth as he was ascending the steps. A police officer told him that there was no smoking on the subway. Morgenbesser pointed out that he was leaving the subway, not entering it, and hadn’t lit up yet anyway. The cop repeated his injunction. Morgenbesser repeated his observation. After a few such exchanges, the cop saw he was beaten and fell back on the oldest standby of enfeebled authority: “If I let you do it, I’d have to let everyone do it.” To this the old professor replied, “Who do you think you are, Kant?” The word “Kant” was mistaken for a vulgar epithet and Morgenbesser had to explain the situation at the police station.