In Act I, Macbeth and Banquo encounter the witches on the heath as the men are leaving a successful battle. The witches hail Macbeth with three titles—Thane of Glamis, Thane of Cawdor, and King. Macbeth already is the Thane of Glamis, so that is his appropriate title. He does not yet know that King Duncan has awarded him Cawdor, as well, since the former thane was a traitor and will be executed. He is naturally taken aback by the witches themselves and by these surprising predictions about his future.
It is this prophecy that Macbeth will rule Scotland one day as its king that spurs all of the action of the play. He immediately begins thinking about killing Duncan and actually commits the murder that same evening, when Duncan stays in Macbeth's castle. It is his confidence in the prophecy that makes him continue to kill to maintain his position (along with his paranoia that he may lose his throne).
Later in the play, he demands even more information from the witches, and three apparitions give...
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