On murder

"FROM my boyish days I had always felt a great perplexity on one point in Macbeth: it was this: the knocking at the gate, which succeeds to the murder of Duncan, produced to my feelings an effect for which I never could account: the effect was-that it reflected back upon the murder a peculiar awfulness and a depth of solemnity: yet, however obstinately I endeavoured with my understanding to comprehend this, for many years I never could see why it should produce such an effect."