Fair is foul, and foul is fair
The rhyming couplet which closes the scene begins with an oxymoron which expresses the contradictory nature of the witches and their corresponding desire to upset the 'natural order' of society. More importantly, Shakespeare later uses it as Macbeth's very first spoken line ("So fair and foul a day I have not seen") to create an eery connection between the witches we have already met and Macbeth's state of mind, indicating despite his heroic status established in Scene ii that he is ripe for their temptation.