This is from the enotes theme on Macbeth and the supernatural, if you follow the link, you'll get the whole section:
Beyond the evil that human ambition can manufacture, Macbeth has a super-natural dimension to it; indeed, the play opens with the three witches stirring the plot forward. Even before his encounter with the three witches, Macbeth finds himself in an unnatural dramatic world on the "foul and fair" day of the battle (I.iii.39). Things are not what they seem. After his first conclave with the witches, Macbeth is unable to determine whether the prophecy of the witches bodes "ill" or "good." He then begins to doubt reality itself as he states that "nothing is / But what it is not" (I.iii.141-142). The prophecy, of course, is true in the first sense but not what Macbeth takes it to be in the second. In like manner, the three predictions made to Macbeth in the first scene of Act IV seem to make him invincible; but the "woods" do march and Macbeth is slain by a man not ("naturally") born of woman.