When Gene returns to Devon fifteen years after graduation, he looks at the tree from which Finny fell and thinks, "The more things stay the same, the more they change." The tree looks vastly changed only because Gene's perspective has changed as he grew up and became an adult. A Separate Peace is the story of this changing perspective, of how things both change and stay the same.
As a story about boys anxious about growing into men, A Separate Peace contains numerous references to change. As the war looms, the carefree joy of summer at Devon turns into the strict discipline of autumn. Finny goes from an athletic youth to a cripple, and then turns Gene from a bookworm into an athlete. Yet though these changes are dramatic to the boys who experience them, when Gene revisits Devon he discovers that the school itself is much the same, almost like a museum. So while all the world felt like it was changing, it was in fact staying the same. Gene himself, however, has continued to grow, and so the very fact that the school stayed the same made it seem to him like it had changed: now the "giants of his childhood" don't seem like giants at all. Gene finds comfort in this: though in the grander scheme of things the world stays the same, because people change they can live harmoniously with their past, and even leave it behind.