Identity
Identity seems to be a really big theme throughout the book. Sometimes Gene wants to be himself and other times he wishes he could be Finny. The scene where this is noticed the most is after Finny gets hurt. Gene puts on all of Finny's clothes and feels like he is Finny. After Finny comes back to Devon, Gene almost transforms into Finny as his best friend becomes kind of his mentor. In another part of the book, Finny tells Gene that if he can't play sports, then Gene is going to have to play them for him. Gene follows in Finny's footsteps and sometimes begins to seem like a clone.
Innocence
I believe that innocence is a big theme throughout the novel. During the summer session, the boys play around and have a good time. They make up new games, play sports, and form a secret club. I think that they lost their innocence the moment Finny fell from the tree. Dealing with the tragedy seemed to make everyone more somber and grown up. When the boys come back for the winter session, they are definitely not as energetic, and they follow the rules a lot more. The realization that they will soon be heading off to war also hardens the boys and turns them into men, especially as they face the difficulties of enlisting early.
Competition and jealousy
Competition may not have been a theme throughout the whole book, but it was undoubtedly a theme in the lives of Gene and Finny. Finny always seemed to be the best. He could make up a new game and win it every time, excel in sports, have no fear of jumping out of the elm tree, and break the school swimming record simply because he felt like he could. At first Gene seems accepting of Finny's athletic abilities, but he eventually become jealous. He begins to believe that Finny is trying to sabotage his grades, which is the only thing he can be better that Finny at. Even though he gets straight A's, Gene still is underneath another student in terms of ranking at the school. Throughout the whole book, Gene always seems like he would rather become someone else than embrace his own abilities.
Rules and Continuity
A Separate Peace mentions rules a lot. During the summer session, Finny almost gets in trouble a couple times by teachers for breaking the rules, but he always manages to get himself out of trouble. When the boys come back for the winter session, rules are mentioned again, but this time they are being enforced. Also, Brinker Hadley is a lover of the rules and law, so the reader hears a lot more about rules during the winter session. Because there are an abundance of rules, the Devon School encourages continuity in the lives of the boys. This means that they try to keep out change and make everything like a routine,