What points of similarity and of difference are there among the air drawn dagger, the voice that cried: "Sleep no more," and the Ghost of Banquo?
Each of these items represent an aspect of Macbeth's subconsciousness and conscience. The air drawn dagger appears to Macbeth before he goes in to kill Duncan. He is clearly hallucinating. This dagger that is covered with drops of blood stem from his "heat-oppressed brain" and represents the difficulty of crossing the line between being an honorable soldier and thane to becoming a traitor and an assassin. The voice that cries "Sleep no more" is once again Macbeth's imagination. This cry occurs after he kills Duncan, and again shows Macbeth's immediate remorse for what he has done. He feels that he will never sleep peacefully again. The last item--the Ghost of Banquo--is another hallucination. In Act 3, Macbeth has had Banquo murdered. And while he outwardly seems to feel no guilt or remores, he cannot suppress images of the evil that he has had done to his former friend and fellow soldier. Each of these images evokes fear and terror in Macbeth, but instead of dissuading him from future violence, seem to create panic and result in even more violence.