An astronomer, physicist, mathematician and programmer are on a train going to a conference.
As they reach $COUNTRY the astronomer sees a black cow and says – see all cows in $COUNTRY are black.
No says the physicist, you mean there is evidence of some black cows in $COUNTRY.
The mathematician says – there exists at least one cow in $COUNTRY that is black on one side.

And the programmer – look! moo-moos !