Three scientists were on a train and had just crossed the border into Scotland. A black sheep was grazing on a hillside. The biologist peered out of the window and said, “Look! Scottish sheep are black!” The chemist said, “No, no. Some Scottish sheep are black.” The physicist, with an irritated tone in his voice, said, “My friends, there is at least one field, containing at least one sheep, of which at least one side is black some of the time."