The Fox and the Grapes

The Fox and the Grapes is a fable by Aesop. It is about a fox who sees some grapes hanging high on a vine. He tries to reach the grapes, but he can't. Since he can't get them, he tells himself that the grapes are probably sour anyway.
This fable shows the idea of cognitive dissonance. The English expression "sour grapes" comes from this fable.