In Act 1, Scene 1, the end of the witches' chant delivers a pronouncement that becomes emblematic for the rest of the play:
"Fair is foul, and foul is fair.
Hover through the fog and filthy air" (I.i.11-12).
Macbeth creates a dynamic theme through the interesting dichotomy between fair and foul. Through the witches' first scene, the playwright suggests that appearances cannot be taken for granted. Events or people who may seem to have the appearance of goodness cannot be trusted to be "fair," just as things that may be "foul" in appearance could very likely be benign or helpful.
The witches' own predictions exemplify the theme of "fair is foul, and foul is fair" as their suggestions of Macbeth's rise to power have all the appearance of being wonderful; they cater to his ambition and pride, and suddenly he sees himself rising to the coveted position of Thane of Cawdor and even King of Scotland. However, in hindsight, the audience or reader can understand that the witches' seemingly "fair" prophecy was actually meant for evil purposes: to tempt Macbeth toward corruption and deceit, leading him to terrible consequences.