What is the meaning of "Fair is foul; foul is fair" in Macbeth?
In Macbeth, the witches' equivocal language is a duality, a riddle, a kind of spell, a critique of the society, and an example of foreshadowing and verbal irony. Really, the statement is a way of showing that the natural order has been inverted, that the bottom of the Great Chain of Being (witches and murderers) will replace the top, the King, God's holy vessel.
Literally, as Enotes "Text in Translation" says, it is "Beautiful is disgustingly filthy, and disgustingly filthy is beautiful." Certainly, this is a commentary on the witches themselves, as they are old hags at the bottom of the socio-economic ladder. By Act III, they will become the King's most trusted advisors. So, it is a foreshadowing and an irony.
- Good is evil; evil is good. The good King Duncan will be replaced by the evil King Macbeth.
- Loyalty is betrayal; betrayal is loyalty. The act of murder will subvert the code of the thanes.
- God is the devil; the devil is god. As the King was considered God's Holy Vessel, his murder will open the door for the devil's chaos.
- Natural becomes unnatural; the unnatural becomes natural. After Duncan's murder, there are earthquakes, horses eating each other, bloody babies, moving forests, and all kinds of sinister and horrifying acts that subvert the natural order.