This is a puzzle named after its creator.

There are four cards lying on a table in front of you. You know for certain that all the cards have one letter marked on one side, and one number marked on the other. Marked on their upper sides are the following:

A

F

3

4

There is a statement which is this: "If a card has a vowel on one side, then it must have an even number on the other side."

How many cards need you turn over, and which cards, to establish whether the statement is true or false?