Shakespeare puts these words in the witches’ chant as part of setting the mise-en-scene for the play’s unfolding. They are saying that the natural order of things is upset, that Nature herself is at odds with her own rules, her own “normal,” and that likewise the human world is out of order and balance. What should be a loyalty to his king becomes, for Macbeth, an unnatural ambition, and a bloody deed rather than the hospitality implied when Macbeth invites Duncan to Dunsinane, “fair” being turned into “foul.” All is out of order. Equivocation, the vagueness of a speech misinterpreted by the hearer, is the upsetting of language, the breakdown between what is meant and what is implied, such as “till Burnim Woods does come to Dunsinane” or “no man born of woman” (because Macduff was born by Caesarean section). What appears as “A” is actually turned upside down to “non-A.” “Fair” becomes “foul.” The witches, with their super-human powers, see that the world is in a state of chaos now. Shakespeare finds a perfect way and the perfect character to utter this phrase and warn the humans, who of course do not heed the warning.