A Separate Peace tells the story of Gene's painful but necessary growth into adulthood, a journey of deepening understanding about his responsibility and his place in a wider world. At the beginning of the novel, the young Gene stands unconcerned, self-absorbed, by the tree that will test his true nature. By the end, Gene has suffered and inflicted suffering, and he has grown into an understanding of his own dark motives. He has lost his innocence and has gained experience.
Gene's innocence at the opening of the novel represents a childlike happiness in conformity. By obeying the rules — occasionally rebelling mildly through sarcasm, "the protest of people who are weak" — Gene maintains a comfortable life, predictable and unthreatening, like Leper's dining room. In Devon, obedient to the rules, approved by the masters, Gene is safe, but he cannot grow. Growth can come only through conflict and struggle, and Gene's conformity acts as a shield against such challenges.
Finny breaks through Gene's shield of conformity, daring him to experience the world more directly, by breaking rules and creating new traditions. With Finny, Gene explores a life unbounded by familiar routines imposed by adults. The freedom exhilarates Gene at times — the first forbidden jump from the tree brings him to a new, heightened awareness of life — but uncertainty nags at him. Finny's whims disturb Gene's comfortable routine of study and proper behavior, habits of obedience that win the approval of adults.
Frightened and threatened by Finny's freedom, Gene reacts like a child — sullen, withdrawn, indirect in expressing objection. Instead of joining Finny wholeheartedly or honestly talking through his feelings (about studying for exams, for instance), Gene suppresses his mixed emotions and turns the new experience of freedom into another kind of conformity: He decides that he must follow Finny's whims without exception or risk losing his friendship. This "all or nothing" thinking, childish in its simplicity, leads Gene to resent Finny and ultimately causes the violent outbreak that destroys a life.
Out of Gene's discomfort arises a dark suspicion: Finny is deliberately drawing Gene away from his studies in order to make him fail. Psychologically, this makes sense to Gene. If Gene is trying to obey the rules in order to win approval — the only validation he really recognizes — then anyone who encourages him to disobey, or follow other rules, must wish him harm. Finny, therefore, must be his enemy. In his own defense, Gene hides his resentment and lets his (seemingly justified) anger burn within him while he single-mindedly pursues his goal to become the best student and so show up Finny.
But Gene's sudden recognition that Finny does not want him to fail proves even more devastating. If Finny is simply being Finny in his free, careless ways, then Gene has lost the meaning of his resentment, the energy that has been fueling his drive to succeed despite his enemy's plotting. Gene's anger and bitterness toward his friend make sense only if Finny is really a lying, manipulating enemy bent on destroying Gene. And Gene's quest for academic excellence makes sense only as means of showing up Finny.
The realization that Finny is not acting as a rival or an enemy, but simply as himself, makes Gene feel insignificant. Like a child who discovers he is not the center of the universe, Gene rages at the insult. On the limb, beside his friend, Gene acts instinctively, unconsciously, and expresses his anger physically by jouncing the limb, causing Finny to fall. The physical release of emotional tension suddenly frees Gene, and he jumps effortlessly, without fear, as he never could before. With the destruction of the threat, Gene's view of the world, and of himself, is restored. The child's self-image of himself as the center of the world is recreated.
Significantly, in describing his actions on the limb, Gene insists not that he bent his knees, but that his knees bent, as if his body were not under his control. Again, Gene takes shelter in a childish, self-centered defense. I did not do it, Gene seems to be saying, my knees did it.
A fall and a tree sharply recall the story of Eden, the Fall of Man, and with it the end of innocence. With Finny's fall, Gene recognizes in himself what Leper condemns as "the savage underneath," the tragic flaw Finny more kindly refers to as "a blind instinct." Gene's sense of guilt, however much he hides it, represents his first pang of morality that needs no outside confirmation. Gene knows what he did, and he knows that he is guilty. For the first time, Gene's sense of right and wrong comes not from bells or exams or masters, but from his own shocked soul. This is the end of innocence, and the beginning of experience for Gene.
But faced with this self-knowledge, Gene rejects it, defensively retreating into his habitual conformity, his comforting sense of himself as an obedient boy. What starts out as a confession and an apology to Finny — a mark of true growth into adulthood and responsibility — quickly becomes an angry rationalization, an attack on Finny that constitutes a second injury. In Brinker's informal Butt Room trial, and later, in the more formal Assembly Room investigation into Finny's accident, Gene persists in withholding the truth, refusing to admit his responsibility. Gene's resistance to the truth is a resistance to growth, a retreat into his passive, conforming past, where he felt safe and good. The revelation of Gene's guilt and his refusal to admit it cause Finny's second fall, the accident that ultimately ends his life.
Only in the friends' last conversation, in the infirmary, can Gene face Finny and freely discuss the fall on Finny's own terms, without rationalization or duplicity. Gene's apology and Finny's forgiveness make it possible for Gene to break out of his self-centered denial. By the end of the novel, Gene has accepted both his own guilt and the gift of Finny's friendship. The experience has helped him to grow into an insightful, responsible, and compassionate adult.