Why is the play "Macbeth" so concerned with images of "fair" and "foul"?

The equivocal words "fair" and "foul" echo throughout "Macbeth", from the very beginning of the play, when the witches say; "Fair is foul, and foul is fair, / Hover through the fog and filthy air." Throughout the play, the contrast between fair and foul creates dramatic tension. It links the witches to Macbeth, represents Macbeth's inner moral conflict, and shows the development of Macbeth's and Lady Macbeth's characters. Thus, Shakespeare uses this conflict between foul and fair to create links between some characters, and to separate others.
Macbeth's first line is; "So foul and fair a…