How do Gene and Finny mirror each other in A Separate Peace?

Shortly after Finny's fall from the tree in A Separate Peace, Gene, consumed by guilt and fear, obeys a strange compulsion to dress like his roommate. He puts on Finny's clothes 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. 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.

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.