“The musculature of the Library of Congress categorization scheme looks like it’s about concepts. It is organized into non-overlapping categories that get more detailed at lower and lower levels — any concept is supposed to fit in one category and in no other categories. But every now and again, the skeleton pokes through, and that skeleton, the supporting structure around which the system is really built, is designed to minimize seek time on shelves. The essence of a book isn’t the ideas it contains. The essence of a book is “book.” Thinking that library catalogs exist to organize concepts confuses the container for the thing contained.