The cow

An economist, a statistician, and a mathematician were traveling by train. After a while, the train entered a new and unfamiliar country. Looking outside, they saw a field, and in the field, a black cow.

The economist turned to his friends and said, "Look! A black cow. I think that in this country, all cows are black."

"Not so fast, my friend," said the statistician. "All you can state is that in this country there exists at least one black cow."

"You're both wrong!" said the mathematician. "Really?" said the other two. "Yes. All you can say is that in this country there exists at least one field in which there exists at least one cow which has one black side."

Moral: be careful of your assumptions.