Very early in the play, Macbeth is confronted with the witches who plant in his mind the idea of becoming King of Scotland. The start of Act 1 scene 3 shows us the witches talking in riddles. We are never quite sure what they mean. That is why they are so dangerous for Macbeth. He has to pick his way through a world where the boundaries of right and wrong no longer seem clear-cut. The witches create a shifting world, and Macbeth's moral sense - his sense of right and wrong - is not strong enough to steer him through safely. This moral confusion is shown in the way that things are no longer what they seem: 'So foul and fair a day I have not seen.'
How can the weather be both foul and fair? We would normally be able to agree that it is one or the other. Even the appearance of the witches contributes to this confusion: 'You should be women And yet your beards forbid me to interpret That you are so.'
Are the witches men or women? Macbeth does not know. He is in a world that disorientates him. The half-rhymes also help to create uncertainty; a full rhyme sounds firm and certain, whereas a half-rhyme offers only a feeling that something is not quite as it should be: 'Good Sir, why do you start and seem to fear Things that do sound so fair?'
The alliteration joins with the half-rhyme to link together two words that are usually opposites: we do not usually fear things that are fair and pleasurable. We assume that Macbeth is showing guilt because he has already had thoughts of killing Duncan to become king.
Banquo is much more direct than Macbeth in questioning the witches. He stays level-headed, whereas for Macbeth, the witches seem to have struck a chord deep inside him. He wants to believe what they are saying to him, so he does not stop to subject their words to rigorous critical scrutiny. He knows that they are 'imperfect speakers'. They only give a shadow of the future rather than the full picture. His mistake is that he completes the picture wrongly.
They seem to speak with confidence. Their words are formulated as statements: 'Thou shalt get Kings though thou be none.'
The problem is that Macbeth accepts these statements as straightforward prophecies of what will happen in the future instead of riddles that have to be interpreted.