The witches' words, "Fair is foul, and foul is fair" is a recurring motif throughout Shakespeare's Macbeth as reality and fantasy merge for Macbeth and Lady Macbeth. For, this line points to the discrepancy between reality and illusion. Macbeth's world is a world where nothing is what it seems. He wonders at a dagger, the ghost of Banquo, and finally the Birnam Forest.
Ironically, what Macbeth imagines cannot happen, does. In Act V, Scene 3, he tells the doctor to bring him his armor after him, declaring,
I will not be afraid of death and bane
Till Birnam Forest come to Dunsinane. (5.3.68-69)
Then, in Act V, Scene 5, a messenger arrives to report that Birnam Wood appears to be moving toward Dunsinane--"nothing is what is not"; "fair is foul, and foul is fair" and the witches' prophesy that Macbeth need fear nothing until "the Wood of Birnam rise" is apparently and fatally true.