Why does Knowles open A Separate Peace with Gene returning to the Devon School 15 years after he graduated instead of having Gene tell the story as it happens?
While the narrative involves Gene Forrester's journey from innocence to experience, it can only be told objectively and insightfully by Gene as a mature adult who looks backward. For, it is not until Gene matures that he becomes an individual in his own right, having been merely an extension of Phineas as a youth; moreover, it is only after his separation from Finny that, with his own identity and life experiences, Gene is able to objectively analyze and reveal his own flaws. Thus, if Knowles is to have Gene provide the narration, he must have Gene narrate in retrospect.
In a Dean Kuntz novel, one character observes, "Perception is reality." Truly, for Gene as a youth, his perception of Phineas was assessed by him as absolutely real when, in fact, it was terribly flawed. For instance, the young Gene admits that he thinks
[T]here was no harm in envying even your best friend a little.
He also believes that Finny feels some animosity to him and that, as Finny shouts during blitzball, "we're all enemies." This envy and enmity grows in the young Gene, clouding his perception. He thinks Finny wants him to fail in his classes, and he enjoys demonstrating his athletic superiority. In an article from Studies in Short Fiction, Ronald Weber writes,
It is Phineas's innocence that Gene cannot endure. As long as he can believe Phineas shares his enmity, he can find relief, but with this assurance gone, he stands condemned before himself and must strike out against his tormentor.
It is then that Gene courts deadly things, revealing in his cruel act of jouncing the limb on the tree his own tragic flaws. However, it is only as an analytical adult that Gene can formulate and assess his motives and actions. After he has served in World War II, Gene comprehends the magnitude of his faults, realizing that such events as the Winter Carnival that Finny fabricated have been only "momentary and illusory"; creating only "a separate peace" because Gene has been at war while in Devon School:
...my war ended before I ever put on a uniform. I was on active duty all my time at school: I killed my enemy there.
Having returned to Devon School, Gene discovers from his memory of events and feelings that it was the private evil in his heart, his courting of deadly feelings, that caused him to, in effect, kill Phineas. This is a realization that he arrives at only after he has been a soldier and has reached adult maturity.