“Because books and their metadata have, until recently, been physical objects, we’ve had to pick one and only one way to order them in defined, stable ways. When Melvil Dewey introduced the Dewey decimal classification system in 1876, it was an advance because it shelved books by topic, making the library’s floor plan into a browsable representation of the order of knowledge itself. But no one classification can represent everyone’s way of organizing the world. You may file a field guide to the birds under natural history, while someone else files it under great examples of the illustrative art and I file it under good eating. The digital world makes it possible for the first time to escape this limitation. Publishers, libraries, even readers can potentially create as many classification schemes as we want. But to do this, we’ll need two things.”