A mathematician, a physicist, and an astronomer are on their way to Scotland. They cross the border and observe a black sheep in the middle of a field. "Look," exclaims the astronomer, "all Scottish sheep are black!" The physicist responds, "No, no! Some Scottish sheep are black!" The mathematician shakes her head, takes a breath and proclaims, "Ladies, all we can truly say is that in Scotland there exists at least one field, containing at least one sheep, at least one side of which is black."