When Finny falls out of the tree, it is certainly an important and significant event in the book, but I feel that it is later on, when Finny is confronted with the news of Gene's treachery that is the climax of the novel. Typically the climax of a book centers around one or more main characters who are confronted with the truth, with a very tough decision, or with an extreme test to their character. All of these statements apply to the part of the novel where Leper is brought in to describe the events at the tree, and when Finny actually fully understands, and can't escape from the fact, that his best friend was the one to destroy his leg, his sporting prospects, his ability to fight in the war, and essentially, his entire life. He is confronted with the truth; Gene has to admit to it also. Up until this point in the novel, Finny has been living in denial, and in a happy, idealistic world where evil didn't exist. When it is revealed to him that it does, and often in those we trust most, it is too much for him--his character cracks. For the first time in the book, he curses, gets angry, and is mean in his language and actions.
So, even though Finny does fall down the stairs, and very dramatic results stem from that event, it is what led up to that stumble that is the actual climax of the novel. Gene, who's been living with guilt over what he did, and who has been struggling with denying it or hiding it, cannot escape the truth any longer, and neither can Finny. That great truth--that, as Gene says later, war is fought in the planes of the heart, between those that we care about, is the climax of the story, because it is the moment of crisis that truly tears the main characters apart. I hope that those thoughts helped; good luck!
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