The quick-fire dialogue and fragmented line structure in this part of the scene denote a sense of frightened urgency in both characters. Macbeth's concern centers on two major areas. First, he believes he has "murder'd sleep." Sleep, he argues, ought to bring physical calm in the same way that prayer soothes the spirit. But in his case, the ability both to pray and to sleep has been cancelled. Macbeth is haunted by the knowledge that he will never again rest easy in his own bed: "Glamis hath murdered sleep, and therefore Cawdor / Shall sleep no more, Macbeth shall sleep no more!" (41–42). Lady Macbeth, refusing to accept such "brainsickly" thoughts, reminds Macbeth of the familiar comparison that "the sleeping and the dead / Are but as pictures." Ironically, she is the one who will be kept from sleeping by the picture of death long after it has left Macbeth's mind.
The second area of Macbeth's concern is the bloodiness of the deed and specifically the fact that his own hands bear witness to the unnatural deed of murder. Again, for Lady Macbeth, blood is only like paint used to daub the picture of death and can be easily washed off. But Macbeth is aware of the deep stain beneath the surface. His capacity for recognizing the grand scale of his action, which foreshadows his later remark that he is "in blood stepped in so far," is missing in Lady Macbeth.
At this point, the knocking begins. Like the beating of the heart in Edgar Allan Poe's short story "The Tell-Tale Heart," the noise is partly the knocking of their consciences and partly an actual exterior knocking. Symbolically, the knocking is the knocking of justice, or of vengeance.
