How does "fair is foul and foul is fair" relate to other lines or events in Act I  of William Shakespeare's Macbeth?

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thanatassa eNotes educator| Certified Educator

In William Shakespeare`s Macbeth, the line `fair is foul and foul is fair` is spoken by the witches on the blasted heath. This phrase refers to a sort of moral relativism or inversion of the moral order, echoing the famous dictum of Protagoras that he could make the worse case appear the better. In Macbeth, the protagonist, who begins as a brave and loyal general to King Duncan, becomes a murderer and brutal dictator. The witches themselves, although foul and evil on the outside, and tempting Macbeth to evil deeds, on the other hand serve the fair purpose of revealing the inward evil or weakness under Macbeth`s good reputation, so that just as Macbeth`s apparent fairness hides inner foulness, so the witches outward foulness is fair in the sense of creating fairness or justice in revealing people`s flaws.