A Physicist, a Mathematician, and a Logician are traveling together by train through Scotland. The Physicist looks out the window, spots a black sheep, and exclaims, "There are black sheep in Scotland". The Mathematician corrects with, we can only claim that there is one black sheep in Scotland. The Logician chimes in, "All we know is that one side of one sheep in Scotland is black".

I got this one form a college professor in 1971. I just Googled it and found that the roles are filled differently in other versions of this story. For examples: Engineer, Physicist, Mathematician or Engineer, Scientist, Mathematician.