Logic Three professors of logic get on a train in Scotland and take a seat. They look out the window and see a black sheep. One of them says: "I didn't know Scotland had black sheep". The second one says, rather pedantically, "You only know Scotland has at least one black sheep." The third one chimes in, even more smugly: "All you know is that there is at least one sheep in Scotland that is black on at least one side." Satisfied with the exchange, they sit back in their seats when another passenger, who overheard them, leans in and says: "excuse me, but shouldn't this joke have started with 'At least three professors...'?"