Subject analysis, then, is based on philosophy and the nature of documents. It is concerned with identifying individual subjects, their parts, and the relationships between those parts. A new and quite different consideration arises when the many individual subjects have to be presented as catalogue, index or bibliography; that is to say, the relationships between these individual subjects, as distinct from the relationships between the parts of any one subject. The intricate pattern of knowledge must be reduced to a very simple arrangement for this purpose. One school of thought goes so far as to support an alphabetical order of subjects, leaving relationships to be shown by cross-reference. This is, in fact, far less simple than it seems, and can only be effective when based on classificatory principles. Pragmatic schemes such as LCSH are inefficient and difficult to use because they are not based on a consistent plan. In any case a classification scheme is the only practicable method of arranging the materials in a library, and if subject headings are used for the catalogue they should derive from the class numbers allocated to documents.