Scottish logic

An astronomer and a physicist were holidaying in Scotland with a Scottish mathematician friend. They were travelling by train into the country when the astronomer spotted a black sheep in a field and said "Oh, that's interesting, all Scottish sheep are black!" To which the physicist replied "No, No, Some Scottish sheep are black"

The Scottish mathematician turns his face to heaven and sighed. He said "I think you'll that, in Scotland there exists at least one field that contains one sheep, at least one side of which is black!"

'taken from Ian Stewart's book "Concepts of Modern Mathematics"