Shortly after Finny's fall from the tree, Gene, consumed by guilt and fear, obeys a strange compulsion to dress like his roommate. He puts on Finny's clothes — even the unconventional pink shirt that was the "emblem" for the Allied bombing of Central Europe — and looks at himself in the mirror. There Gene sees he has become Finny "to the life." The physical resemblance Gene senses, brings on a surge of Finny's own unique spirit within him. Unexpectedly, Gene feels free, daring, confident — just like Finny. For a moment, Gene has become Finny's double.
In a sense, Gene and Finny have been each other's doubles since the beginning of the novel. In the first description of the boys standing together by the tree, the narrator makes clear that they resemble each other physically to a remarkable extent. Their heights and weights are nearly identical, although Finny weighs about ten pounds more than Gene. But the crucial ten pounds, Gene notes with envy, are distributed evenly over Finny's body. Finny, therefore, does not look like Gene with extra weight. Instead, next to Gene, Finny's entire physique looks more filled out, somehow more striking. This weight difference, "galling" to Gene, seems to prove that Finny stands as the larger, more substantial, somehow more generous, of the two. For Gene, then, Finny represents another version of himself, only better and more powerful.
Without even trying, Finny shows Gene up in the most basic, physical way. Even more frustrating, Finny accepts his shorter than average height without difficulty, while the unconfident Gene tries to embellish his own physical stature by adding a half-inch. When Finny hears this, he virtually cuts Gene down to size by attesting flatly that they are the same height. Gene cannot lie about himself, it seems, because his other self — as like him as his shadow — will speak the truth.
The "shadow" side of the double expresses Gene's mixed feelings about Finny from the start. Some critics have identified Finny as Gene's "Doppelganger," another self, wild and uncontrollable, that Gene loves but feels he must destroy. Gene is the good boy, the theory explains, the student who wants to obey, but is prevented by dark forces beyond his control, represented by Finny.
Throughout the novel, Gene's preference for an orderly life is disrupted by Finny's whims, impulsive and dangerous. As much as Gene enjoys these occasional thrills, he feels threatened — both academically and personally — by Finny's freedom. At one point, Gene even becomes convinced that Finny's outings and forbidden jaunts are a deliberate attempt to sabotage Gene's plans to become the valedictorian. Since Gene's academic ambitions are so close to his heart, so crucial a part of his self-image, the suspicion horrifies and angers him.
Given this tension, Gene's instinctive jouncing of the limb might represent a kind of self-defense: an unconscious attempt to destroy, or at least to cripple, a dangerous, uncontrollable part of himself — his shadow self. Gene's action, then, takes away Finny's power to disrupt Gene's orderly progress towards conventional adulthood. After the fall, Gene does not have to fear the consequences of Finny's unthinking action. The irony, of course, is that Gene's own unthinking action will have terrible consequences of its own.
As one a scholar and the other an athlete, Gene and Finny have been complementary selves — their abilities completing each other in friendship. After the fall, Finny determines to make the union of selves real in Gene, by training him to excel in sports as well as academics. For a superb athlete like Finny, the loss of physical ability represents an essential loss of self, a pain expressed in his uncharacteristically bitter remark, "I've suffered!" Yet Finny trains Gene with grace and good humor, delighting in his physical progress, generously sharing the dream of the 1944 Olympics. In fact, Finny trains Gene as enthusiastically as if Gene were a part of himself. Gene feels Finny's identification, and responds in turn by becoming, in his own way, a part of Finny.
The dramatic revelation of Gene's part in Finny's fall breaks the friendship temporarily, bringing about a nightmarish loss of self in Gene, but their reunion makes possible a new, more complete life. After Finny's death, Gene senses a new peace in himself, a self-confidence that enables him to cope with minor annoyances, like the condescension of Brinker's father, as well as great challenges, like service in the war.
By the end of the novel, Gene has fulfilled the earlier promise of the image in the mirror. He has killed his "enemy" — a narrow, fearful self — and filled himself with Finny's self-confidence and freedom. Gene has become a bigger and better self through friendship with his uncontrollable, unpredictable double, Finny.