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The theme "fair is foul, and foul is fair" appears first in Act One when the witches plan to meet Macbeth.
Fair is foul, and foul is fair.
Hover through the fog and filthy air. (I.i.11-12)
"Fair is foul, foul is fair" means that things that appear to be good ("fair") are often actually bad, and things that at first look bad can actually be good. This paradoxical statement seems to be self-contradictory: how is it possible something could be bad and good at the same time? We find it is possible, and it is a truth Shakespeare weaves throughout the play.
Note Macbeth and Banquo's discussion in Act One, scene three.
So foul and fair a day I have not seen. (39)
This seems an impossible statement. However, Macbeth is saying that the weather is bad ("foul"), but the outcome of the battle (their victory in the fight) makes it good ("fair").
By the start of Act Four, Macbeth has murdered his King, and his best friend, and will soon order the murders Macduff and his family. It is, in fact, Macduff who is the object of the witches' first prophecy when Macbeth visits them for help.
In Act Four, scene one, the witches have created a spell that will give Macbeth misleading information—information that will at first glance seem positive ("fair"), but will eventually prove to be Macbeth's undoing ("foul") because he takes what he hears and sees at face value. (He also has a "false sense of security" that will create in him a mindset that he cannot be defeated.)
The first apparition is an "armed head" (which means a head with armor—a helmet—on it). The apparition delivers a prophecy:
Macbeth! Macbeth! Macbeth! Beware
Beware the Thane of Fife... (78-80)
The "armed head" represents Macduff. The warning seems unimportant to Macbeth. He does, however, have has his doubts about Macduff because he did not attend Macbeth's feast when called.
The second apparition is a "bloody child." The prophecy it delivers is:
Be bloody, bold, and resolute; laugh to
The power of man, for none of woman born
Shall harm Macbeth. (87-90)
This piece of information is especially enticing to Macbeth ("fair") because he is convinced that anyone that is born by a woman (which he sees as the only way a man may be born) cannot harm him. This is misleading ("foul") because the prophecy literally means that the only person that can defeat Macbeth is a man who was not born in the traditional way. Macduff was a Caesarean-section baby (C-section); this means he was not born in the traditional way, and he, therefore, can kill Macbeth.
Macbeth does not consider any of this, however, for it is trickery on the part of the witches. He should know better than to trust the witches because they serve the devil; but he is naive enough to believe they speak the truth. At first he dismisses Macduff's potential threat; but then—just to make sure—he decides to kill Macduff anyway.
The third apparition is the sneakiest. It shows "a Child Crowned, with a tree in his hand," and intones:
Macbeth shall never vanquish’d be until
Great Birnam Wood to high Dunsinane Hill
Shall come against him. (103-105)
Macbeth is told he cannot be defeated unless the woods move. This seems impossible ("fair"); however, when Malcolm ("a Child Crowned") attacks, his men are carrying tree branches, and Macbeth is defeated ("foul").