At the beginning of the play, the witches say, "Fair is foul, and foul is fair."  Later in Scene 3, Macbeth refers to this line when he says, "So foul and fair a day I have not seen."  The line is a paradox that means that things are not always what they seem.  Macbeth appears to be a loyal soldier to Scotland; however, in the next act, he murders the king whom he claims to love so that he can get power for himself.  The line is a statement of one of the themes of the play, and Macbeth brings this up again when he and Banquo meet the witches.  This reiterates the fact that Macbeth will read into the witches' prophecy in the way that he wants even though their words may mean something different.