'fair is foul, and foul is fair'
the witches say this in order to create a sense of confusion, showing us that thing's are not always as they seem. the use of PARADOX here highlights the power of the witches, where they give macbeth "fair" prophecies
but will end up with "foul" consequences
the witches.it also acts as a sort of summary to the play showing whats to come. shakespeare uses the phrase to show what is considered good is in fact bad and whats considered bad is actually good