Themes are the fundamental and often universal ideas explored in a literary work.
The central relationship in the novel—that between Finny and Gene—involves a complex dynamic of seeking to establish, yet being uncomfortable with, identity. Early in the book, the boys’ relationship seems fueled, in part, by Gene’s envy and resentment of his friend’s dominating spirit. As Finny demonstrates his physical prowess, Gene feels the need to accentuate his academic prowess. Finny’s fall from the tree, however, apparently purges Gene of his darker feelings and steers their relationship in a different direction so that codependency rather than envy characterizes it. The scene immediately following the fall symbolizes this evolution, as Gene dresses in Finny’s clothes and sees himself as looking exactly like him. From this point on, he and Finny come to depend on each other for psychological support. Gene plays sports because Finny cannot, allowing Finny to train him to be the athlete that Finny himself cannot be. This training seems an avenue for Finny simply to live vicariously through Gene. But Gene actively welcomes this attempt by Finny, for just as Finny derives inner strength from fulfilling his dreams through Gene, so, too, does Gene find happiness in losing his own self (which he seems to dislike) in Finny’s self (which he likes very much).
Thus, the boys’ relationship becomes a model of codependency, with each feeding off of, and becoming fulfilled by, the other. This codependency preempts the development of their individual identities, perhaps dangerously: by living within their own private illusion that World War II is a mere conspiracy and continuing to believe that Gene (and Finny through him) will go to the Olympics and that the outside world can never curtail their dreams, the boys are refusing to grow up and develop their own ambitions and responsibilities. Not even Finny’s death, though it separates them physically, can truly untangle Gene’s identity from Finny’s—he feels as though Finny’s funeral is his own. In a sense, the reader realizes, the funeral is indeed Gene’s own; so much of him is merged with Finny that it is difficult to imagine one boy continuing to exist without the other. It is perhaps only his ultimate understanding that Finny alone had no enemy that allows the older Gene to reestablish a separate identity—one that he considers, however, inferior to Finny’s.
A Separate Peace takes place during wartime and is emphatically a novel about war—and yet not a single shot is fired in the course of the story, no one dies in battle, and only the unfortunate Leper even joins the military before graduation. Instead, Knowles focuses on the war within the human heart, a war that is affected by the events of World War II but exists independently of any real armed conflict. For Knowles—or at least for his narrator, Gene—every human being goes to war at a certain point in life, when he or she realizes that the world is a fundamentally hostile place and that there exists in it some enemy who must be destroyed. The novel implicitly associates this realization of the necessity of a personal war with adulthood and the loss of childhood innocence. For most of Gene’s classmates, World War II provides the catalyst for this loss, and each reacts to it in his own way—Brinker by nurturing a stance of bravado, for example, and Leper by descending into madness.
Gene himself, though, states that he fought his own war while at Devon and killed his enemy there. The obvious implication is that Finny, as the embodiment of a spirit greater than Gene’s own, was his enemy, casting an unwavering shadow over Gene’s life. One might alternatively interpret Gene’s statement to mean that this enemy was himself, his own resentful, envious nature, which he “killed” either by knocking Finny from the tree or by obtaining forgiveness from Finny for doing so. In either case, the overall theme is clear: all humans create enemies for themselves and go to war against them. Everyone, that is, except Finny, the champion of innocence, who refuses to believe that anyone could be his enemy. In a sense, Finny’s death is inevitable: his innocence makes him too good for the war-torn and inimical world in which the rest of humanity lives.