Nevertheless, however much he reasons, Macbeth cannot reconcile the fact of the truth of the first prophecy with his intense and unnatural fear, or what he calls his "horrible imaginings." He admits to being so shaken by the news that he feels that his reason has been taken over by his imagination. 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.
