What does "proceeding from the heat oppressed brain?" Mean?
No doubt this is an incorrect self-diagnosis of his mental condition based on a faulty understanding of human physiology and psychology. Macbeth is apparently thinking that his turbulent emotions have caused his blood to become unnaturally heated and the warm blood entering his brain might be causing him to have hallucinations, in one of which he sees a floating dagger leading him toward Duncan's chamber. He is not completely sure whether the dagger is real or imaginary. This is the first of many hallucinations Macbeth will be experiencing once he has committed himself to murdering the King. The dagger Macbeth thinks he sees may have been "real" in the sense that Shakespeare may have actually arranged to have a dagger suspended from a dark string dangling in front of the actor playing Macbeth and apparently leading him on to commit his treasonous crime.