Macbeth is a tragic play. There are several key occasions which lead to the tragic hero's downfall: firstly when Macbeth matches the three witches in Action 1 Picture 1, second when he makes a decision to kill Ruler Duncan in Work 1, and lastly the order of the killing of Macduff's better half and children just before Lady Macbeth kills herself. These three moments are crucial in moving the play and Macbeth's demise forwards, and Shakespeare uses these very intelligently.
At the start the witches play a massive part in the play. They anticipate Macbeth's future and Elizabethan would have presumed that they cast a spell on the play which evolved everything. The lines 'Fair is foul, and foul is reasonable' in Work 1 Picture 1, means two related things in general. First, this means that things that are good can be bad and things that are bad can be good.
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