In "Macbeth", Act I, Scene I, the witches declare, "Fair is foul, foul is fair." This statement is central to one of the themes of the play, appearance versus reality.
In this scene, the witches have seen that the battle has finished, and they are to meet Macbeth in an open field. They are creating a somber atmosphere over what seems a great victory. So foul, that means evil, and fair that means good may turn one into the other. Although victory, and the glories that Macbeth is about to receive may seem fair, good, they may turn into foul, evil.
