In A Separate Peace, because the boys were at a school and not yet physically involved in the war, Devon school was a place of peace. Even Gene noted that the students probably reminded the faculty of what peace was like. They were young and still somewhat carefree. However, the irony is that despite the fact that these would be, symbolically, the last years of innocence and childhood, the boys were being groomed for entrance into WWII and therefore, it was always on their minds. It was therefore a peaceful time when the faculty allowed the students to indulge in their innocence but it was also a tense time as the boys approached the age when they could be drafted.
Finny's attitude about the war and his general naivety exemplified the true spirit of peace. In fact, Gene's jealousy of Finny had everything to do with Finny's ability to be so youthful and carefree at a time when Gene (and other boys) were faced with the responsibility of becoming men about to enter a war.
One could interpret the aspects of peace in different ways as it relates to the war, the school, and the relationship between Finny and Gene. One angle is that Gene never really understood why he was jealous of Finny (and why he pushed him) until he was much older. One interpretation is that Gene was so afraid and angry about maturity and entering a war that he had to completely separate himself from any sense of childhood innocence. And Finny was the spirit of youth at Devon; he was that childhood innocence. Gene felt he had to separate himself from that peace.
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