[Aside] . . . Present fears
Are less than horrible imaginings;
My thought, whose murther yet is but fantastical,
Shakes so my single state of man that function
Is smother'd in surmise, and nothing is
But what is not.
Look how our partner's rapt.
Macbeth:Macbeth Act 1, scene 3, 137–144
[Aside] If chance will have me king, why, chance may crown
Without my stir.
No sooner do three witches proclaim Macbeth future king of Scotland than Macbeth starts thinking up bloody business. While his ambition gets ahead of his conscience, Macbeth is still frightened by his own "imaginings" of murdering the present king, Duncan (who's a relative, no less). The murder is as yet merely a fantasy ("fantastical"), but the fantasy is powerful enough to "smother" Macbeth's "function"—his normal grip on reality. For Macbeth, "nothing is/ But what is not": nothing is real to him but what is imaginary.
Macbeth's weak defense against his imagination is the hope that if destiny ("chance") will have him to be king, then destiny will do the dirty work, and he won't have to lift a finger. Chance may crown him without his stirring in his own service. But notice the subjunctive mood of "may": chance may take care of the business, but then again, Macbeth may still have to do it himself.