4. Classification versus Cataloguing.—-One of the principal causes of confusion in the classification of knowledge, as contained in books, is the failure to distinguish between classification and cataloguing. Classification may be defined as the art of placing one copy of a book in that part of a library where all similar books on the same subject are kept. It is physically impossible to make the same copy of one book act as a cross- reference from a standpoint to a concrete subject, or vice versa, because it can only occupy one place at a time, and no one would dream of using ten, twenty or one hundred copies of the same book in order to place one at every subsidiary subject of which it treated. The problem of classification is thus complicated by the physical necessity, imposed by the form of books, of choosing only one place in which to class a given book ; and, naturally, it is to be supposed that the place chosen will be the most useful, or one most likely to be resorted to by the largest number of enquirers. With cataloguing there is no such physical difficulty. The entry of a book can be multiplied to almost any extent ; a book can be shown under every imaginable subject ; cross-references of all kinds can be easily made ; and no question can arise of increasing the number of books or the size of the library. It is largely due to a misunderstanding of the fact that a catalogue is to a great extent but an index to a scheme of classification, that so many persons obtain the idea that books in an actual arrangement on the shelves should appear at every possible class, phase, form or subdivision. It is very important that this fact should be borne in mind in every question relating to exact classification, because no scheme, however detailed, can be made to serve the purpose of a catalogue in all