Thing in the Attic (version 2)

In the far future, the descendants of humanity have evolved into creatures of the treetops, small and furred, with tails for balance. They live in a world where the legendary Giants are nothing more than children's stories, dismissed by the educated elite as primitive myth. When arch-doubters like Honath are cast out of their treetop society for their skepticism, they face a terrible sentence: exile to the "hell" below, the forest floor where no tree-person can survive. What they discover there will shatter everything they believed about their world, their history, and themselves. Blish weaves a haunting tale about the nature of faith, the weight of certainty, and the terrible beauty of having your doubts vindicated in the most horrifying way possible. This is science fiction at its most philosophical, asking uncomfortable questions about what we believe, why we believe it, and whether the truth is something we can actually survive knowing.