A mathematician, a physicist, and an astronomer were traveling north by train. They had just crossed the border into Scot- land, when the astronomer looked out of the window and saw a single black sheep in the middle of a field.
“Scottish sheep are black,” he remarked.
“No, my friend,” replied the physicist, “Some Scottish sheep are black.”
At this point the mathematician looked up from his paper and said: ”In Scotland, there exists at least one field, in which there exists at least one sheep, at least one side of which is black.”