The classification system works, sort of. But... It comes a moment after going to the library a number of times when you realize that human knowledge is _not_

The classification system works, sort of. But... It comes a moment after going to the library a number of times when you realize that human knowledge is _not_ hierarchical, a tree - it is a graph. And then you understand why it is a monumental task to make the classification to match people's expectations. You can't map a graph onto a tree.