The document summarizes the Dewey Decimal System, which was devised in 1873 by Melvil Dewey to classify nonfiction books in libraries. It divides books into 10 main categories based on their subject. Each category is explained using a caveman analogy, imagining how a caveman might categorize information based on questions like "Who am I?" or "How can I understand nature?". The categories range from general knowledge to geography and history.