And nothing is
But what is not.

– William Shakespeare

Macbeth, Act 1, Scene 3. Macbeth dreams of becoming King of Scotland ever since the Witches prophesied it. So caught up is he in this fantasy, he thinks of nothing else. He can only think of “what is not,” or what doesn’t exist – yet! This sentence also echoes the theme of appearance vs reality, since it suggests that things that appear to be are actually not. Macbeth’s words are ironic, since his much-lauded loyalty to King Duncan turns out not to be real and he murders the King.