I have suggested that the three witches in Macbeth are expressing their own perspective because they themselves are about as foul as three women can get. To them whatever is fair is foul because it makes them feel envious and spiteful, whereas whatever is foul seems fair because it is like themselves. This expresses an important and readily observable truth about human nature. People judge others by themselves. Wicked people are always attracted to wicked people, and they tend to despise people who are not as greedy, dishonest, mean-spirited, and treacherous as themselves. Birds of a feather flock together. You can see it all around you. You can see it in your school if you observe. It is a misfortune for a decent, intelligent, honorable boy or girl to fall in with a bad crowd, because they will do their best to corrupt him or her.
In Shakespeare’s King Lear, the Duke of Albany has a quarrel with his wife Goneril and tells her:
Wisdom and goodness to the vile seem vile.
Filths savor but themselves. (4.2)
In Shakespeare’s The Tempest, Caliban, who is a foul, vicious character, meets up with newly arrived Stephano and Trinculo, who are a couple of drunken good-for-nothings, and thinks they are semi-divine creatures. He asks Stephano, “How does thy Honor? Let me lick thy shoe." The whole of Act 2, Scene 2, illustrates how fair is foul and foul is fair and how birds of a feather instinctively flock together.
I don’t believe the witches in Macbeth are predicting what is going to happen in the play but are expressing the feelings of three loathsome creatures about the world they inhabit. Whatever is bad is good and whatever is good is bad—from their warped point of view. They are consumed with hatred because of their ugliness and wretched condition. They are always together.
In our contemporary world a beautiful girl or a handsome boy will often find that some people seem to take an instant dislike to them for no apparent reason. There is a reason, all right.