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"Fair is Foul, Foul is Fair" are the very first words uttered by the witches in the play Macbeth in Act 1 scene 1. They strike the key-note of the play, in which the values are all topsy-turvy and in which the chief protagonist, like Milton's Satan make evil their good.
The perversion of values and ideals is apparent in their own deformity - the witches should be women but their beards deny it. Their doctrine reverses the natural order of things.
"Fair is Foul, Foul is Fair" is the Satanic principle of 'Evil be thou my good'. It echoes in Macbeth's first words "So foul and fair a day I have not seen". He goes on to adopt it in order to gain the throne, and then finds that he cannot escape from it.
The confusion of 'fair' with the 'foul' is the play's constant theme. It is emphasised by the heavy irony of Duncan's misjudgement of the two Thanes of Cawdor and, in contrast, by his son's elaborate testing of Macduff.
The first part of the witches' jingle "Fair is Foul" is true because what should have been 'fair' - kingship - becomes 'foul', polluted by the means by which it was obtained.
These words "Fair is Foul, Foul is Fair" are a stroke of genius. They suggest that the the world of this play is the world of distorted values. Macbeth is guided by deceptive apparitions and hallucinations. His moral sense becomes as confused as are his physical senses when he cannot distingush between the real and the unreal dagger. It is the latter that directs him to the murder - he follows unreality.
Nature is foul from the beginning to the end and the foul in nature closely corresponds to the foul or evil in the heart of its hero. Macbeth the hero turns Macbeth the villain and commits the most foul deeds. nature, too, is foul as if in harmony with human guilt and villainy.
the witchs are saying this lines,it meanse that what is good for other not good for us and what is not good for other is good for them.i think this is the simplest anser of this question.