The witches begin by preparing us for contradictions, and then produce contradictory prophecies throughout the play that drive Macbeth mad.
We are told in the beginning that “Fair is foul, and foul is fair” (Act 1, Scene 1, p. 8). What’s good is bad, and what’s bad is good. The witches make good on this promise by prophesizing Macbeth that he will become king, even though he isn’t named king immediately and has to kill the king and drive off his rightful heirs.
The most striking example of contradictory language is in the final set of prophecies. Macbeth is told through a series of visions of dangers he will face, but they seem so absurd that Macbeth shrugs them off. For example, first they tell him to “Beware Macduff” and then tell him that no man can hurt him.
Be bloody, bold, and resolute; laugh to
The power of man, for none of woman born
Shall harm Macbeth. Descends.(90) (Act 4, Scene 1, p. 60)
As it turns out, Macduff was born by caesarian. Technically, he is not “of woman born” and therefore the prophecy is true. But since Macbeth was told both to beware Macduff and that he cannot be harmed, he chooses to believe that he is safe and acts as if he is, leading to his doom. When he meets Macduff in battle and finds out of the untimely womb-ripping, he gives up.