Are you refering to quotes such as "Fair is foul and foul is fair" with the term "paradox quotes"? It is important to identify the number of quotes throughout the entire play that are used by a variety of characters that refer to the ideas suggested in this first quote, uttered by the witches together at the end of Act I scene 1. It serves to introduce us to the crazy, topsy-turvy world of the play where what seems to be fair is actually foul and vice versa.
Key to think about is the way that Lady Macbeth seems to be on the side of the witches implicitly through the advice that she gives to her husband as she plots the death of Duncan. In Act I scene 5, for example, she almost seems to rebuke her husband for the way in which his face can be so easily read, then going on to give him some advice that could have come straight from the witches themselves:
...look like the innocent flower,
But be the serpent under't.
Appearances are often so different from reality, and as Lady Macbeth and Macbeth anticipate the arrival of Duncan at their castle, Macbeth is told by his wife to deliberately resemble a loving servant of their king whilst secretly plotting his murder behind this guise.