“Since multiplicity of helpful order among specific subjects is a fact independent of library classification – a fact to be reckoned with in arrangement – how are we to provide for it? It is a case of arranging concrete materials – books and other kindred materials – in such a way that one kind of arrangement presents itself to one person and another kind to another person. To secure this by pressing a button is obviously possible only in the world of fancy; it is not possible in the world of reality.”
Ranganathan, Philosophy of Library Classification (1951)
Folksonomias? Google?