Here is a card with the four statements that you see, and no others. Assume that each statement on the card is either true or false; then find and identify each false statement. (Refer to them as a, b, c, d if you want, to speed things up.)
On this card exactly one statement is true.
On this card exactly two statements are true.
On this card exactly three statements are true.
On this card exactly four statements are true.
On this card exactly two statements are true.
On this card exactly three statements are true.
On this card exactly four statements are true.
The statements are all mutually contradictory; it is not possible that more than one could be true.
They could all be false, or the first could be true and the others false.