What does "nothing is but what is not" mean in Act 1, Scene 3 of Macbeth?

Macbeth has just learned that he is indeed the new Thane of Cawdor from Ross, and this makes it seem as though the Weird Sisters correctly predicted the future (when all they really did was tell him something he did not yet know). They told him that he was Thane of Glamis (his old title) and Thane of Cawdor (his new title) and that he would be king someday, and now that he learns that he is Thane of Cawdor, he begins to imagine being king, too. To this end, he says,

Two truths are told
As happy prologues to the swelling act
Of the imperial theme. (1.3.140–142)

He seems to begin to consider what it would take for him to become king, perhaps realizing that Duncan—his king, cousin, and good friend—would have to die. It may be this "horrid image" that makes his "heart knock at [his] ribs" when he imagines his future (1.3.148, 149). The thoughts he is having about becoming the king—perhaps even about killing the king—"shake" him so much "That function is smothered in surmise / And nothing is but what is not" (1.3.154–155).

In other words, he is so busy considering the possibilities of what could be but what is not yet that his thoughts really serve no function in the here and now. He can only think on things that are not yet true or real—such as him becoming king—and so, to his brain, the only thing that "is" is what "is not" in reality.