The line “Nothing is, but what is not” is ambiguous. The expression could indicate confusion between the world we think of as real and the world of dreams, a neat summary of a confused mind. But how confused is Macbeth at this point? If he is capable of arguing that the prophecies are neither evil nor good, he is capable of accepting that nothing that exists has any existence or meaning. This interpretation could open Macbeth to dangerous and unjustifiable deeds. If he can make himself believe that “Nothing is, but what is not,” then Macbeth’s respect for order, for hierarchy, for the King, is also nullified. He can, literally, get away with murder.
I deconstructed the sentence like this:
(1)Nothing is, (2)but what is not.
( 1 ) Nothing is real to me, (2) but what is not real.
(1) i.e. nothing is real to me, but my imaginings/fantasies of murdering the king.
So: only my fantasies are real to me.
therefore “nothing that exists can have any meaning”
very nihilistic, which is actually another theme in Macbeth.