The weird sisterhood of the three witches introduces the theme of the battle between good and evil in the very opening scene of Shakespeare's Macbeth. The witches appear on ' a desert place ' in the midst of ' thunder and lightning ' after having performed an incantation while a battle is going on elsewhere in King Duncan's Scotland. The ' weird sisters ' speak enigmatically in a highly stylized mode, in Trochaic tetrameter, referring to some ' hurlyburly ', some battle being ' lost and won ', and a meeting with Macbeth ' upon the heath ' before sunset. The witches leave, responding to their pets, with a choral proclamation in a rhymed couplet that strikes the key-note of the play:
Fair is foul, and foul is fair:
Hover through the fog and filthy air.
' Fair is foul, and foul is fair ' is a sinister formula given by the witches, a formula couched in Chiasmus suggesting an apparent syntactic inversion which implies a deeper moral inversion in the world of man. If Fair and Foul are equable and interchangeable, they are no longer distinguishable, and that shows a state of profound moral chaos. Furthermore, the witches propose to meet Macbeth soon, presumably alone on a desert place again, thus rousing our suspicion if Fair Macbeth is foul too. If the witches are Foul and if they target Fair Macbeth to be their victim of temptation, the battle between good and evil obviously come to the foreground of the play.