Dichotomy and Equivocation Theme
"Fair is foul, and foul is fair / Hover through the fog and filthy air" (I i 10-11). The first scene of the first act ends with these words of the witches, which Macbeth echoes in his first line: "So foul and fair a day I have not seen" (I iii 36). In a similar fashion, many scenes conclude with lines of dichotomy or equivocation:
"Hear it not, Duncan; for it is a knell / That summons thee to heaven or hell" (II i64); "God's benison go with you, and with those/ That would make good of bad, and friends of foes" (II iv 41-42).
Such lines evoke an air of deep uncertainty: while polarities are reversed and established values are overturned, it is entirely unclear as to whether the dichotomous clarity of "heaven or hell" trumps the equivocatory fogginess of "fair is foul, and foul is fair." Thus, for Macbeth, this translates into an uncertainty as to whether the prophecies are believable. It seems that Birnam Wood will either come to Dunsinane Hill (a supernatural event) or it will not (a natural event); but the actual even turns out to be neither here nor there, as the Wood figuratively comes to Dunsinane.