The "swelling act" is the act of Macbeth
's becoming king.
is rewarded by King Duncan
and given the title of Thane of Cawdor, he realizes that the first two predictions of the witches
have come true. He has been Thane of Glamis, then is made Thane of Cawdor. Now, he is tempted by the idea that the third prediction, this "swelling act" of being "King hereafter":
Two truths are told,
As happy prologues to the swelling act
Of the imperial theme. (1.3.130-133)
As he imagines himself becoming King of Scotland, Macbeth starts to have murderous thoughts about King Duncan, ideas about the "swelling act" of being king himself. Further, Macbeth considers how his thoughts disturb him so much that he begins to wonder who he really is now. Macbeth worries that he is consumed by his speculations because he finds himself thinking about things that do not yet exist and how he can contribute to bringing them about. A man of action, Macbeth marvels at how he now is consumed by matters that are not real, but only speculative: "And nothing is but what is not" (1.3.145). Here again, appearance and reality are confused.