This scene is pivotal in the plot's development because this is when Macbeth truly takes hold of the ambitious desires which will lead him to commit several murders in order to claim what he believes is rightfully his.
The witches appear and cast some predictions for Macbeth. They tell him that he will be Thane of Cawdor and king in the future, and at first he questions how this could possibly be true. After all,
The thane of Cawdor lives,
A prosperous gentleman, and to be king
Stands not within the prospect of belief,
No more than to be Cawdor. (1.3.76–79)
Yet later in this scene, when Angus tells Macbeth that the current Thane of Cawdor is "under heavy judgement" and deserves death, he also tells Macbeth that he's gained a new title: Thane of Cawdor.
This brings truth to one of the witches' predictions, and Macbeth has a choice to make here. He could proceed with caution or count it as an odd coincidence that one of the predictions of these odd witches has come to fruition. But he doesn't...
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