A mathematician, a physicist, and an engineer were travelling through Europe by train, looking out the window.

Shortly after crossing the border into France, they saw a black sheep through the window of the train.

"Aha," says the engineer, "I see that all French sheep are black." The physicist, of course, corrects this gross generalization.

"Come, come," she says, "all we can say for sure is that there exists at least one black sheep in France".

The mathematician, of course, corrects the physicist. "No," says the mathematician, "All we know is that, there is at least one sheep in Scotland, which is black on at least one of its sides!"