Out, Out, brief candle!
Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage
And then is heard no more: it is a tale
Told by an idiot, full of sound and fury,
Macbeth, Act V, Scene 5
When Macbeth learns of his wife's death, he cries out the above lines, which can be used as a clue to the meaning of the novel or to the structure of the novel. Certainly Faulkner plays with the idea that life is nothing but a shadow. The word shadow appears continually throughout Quentin's section, and it also occurs frequently throughout the rest of the novel.
The implication that life is a shadow is used also by Faulkner to suggest that the actions performed by modern man are only shadows when compared with the greater actions performed by men of the past — that modern man is only a shadow of a being, imperfectly formed and inadequate to cope with the problems of modern life. Man is forced to commit suicide, as Quentin does, and while performing this destructive act, he sees his shadow rising up from the water beneath him. If man does not take his own life, then he is either a materialist like Jason, who values nothing except money, or else he is an "idiot" like Benjy, who can see only shadows of life.
If life "is [only] a tale! Told by an idiot," we then have our justification for having the first part of the story told through the mind of thirty-three-year-old Benjy, for the story that Faulkner tells is indeed full of all types of sound and fury.