A biologist, a computer scientist and a mathematician are on a train, heading from England to a conference in Edinburgh.
Once over the border, they see a black sheep in a field.
“Aha!” thinks the biologist, “Scottish sheep are black!”
“Aha!” thinks the computer scientist, “There are sheep in Scotland, at least one of which is black!”
“Aha!” thinks the mathematician, “There exists a country called Scotland which contains at least one sheep, at least one side of which is black!”
Quora Magazine