714 THE PRINCIPLES OF SCIENCE. [CHAP. 



Index Classifications. 



Under classification we may include all arrangements of 

 objects or names, which we make for saving labour in the 

 discovery of an object. Even alphabetical indices are real 

 classifications. No such arrangement can be of use unless 

 it involves some correlation of circumstances, so that 

 knowing one thing we learn another. If we merely 

 arrange letters in the pigeon-holes of a secretaire we 

 establish a correlation, for all letters in the first hole will 

 be written by persons, for instance, whose names begin 

 with A, and so on. Knowing then the initial letter of 

 the writer's name, we know also the place of the letter, and 

 the labour of search is thus reduced to one twenty-sixth 

 part of what it would be without arrangement. 



Now the purpose of a catalogue is to discover the place 

 in which an object is to be found ; but the art of cataloguing 

 involves logical considerations of some importance. We 

 want to establish a correlation between the place of an 

 object and some circumstance about the object which 

 shall enable us readily to refer to it ; this circumstance 

 therefore should be that which will most readily dwell in 

 the memory of the searcher. A piece of poetry will be 

 best remembered by the first line of the piece, and the 

 name of the author will be the next most definite circum- 

 stance ; a catalogue of poetry should therefore be arranged 

 alphabetically according to the first word of the piece, or 

 the name of the author, or, still better, in both ways. It 

 would be impossible to arrange poems according to their 

 subjects, so vague and mixed are these found to be when 

 the attempt is made. 



It is a matter of considerable literary importance to 

 decide upon the best mode of cataloguing books, so that 

 any required book in a library shall be most readily 

 found. Books may be classified in a great number of 

 ways, according to subject, language, date, or place of 

 publication, size, the initial words of the text or title-page, 

 or colophon, the author's name, the publisher's name, the 

 printer's name, the character of the type, and so on. Every 

 one of these modes of arrangement may be useful, for we 

 may happen to remember one circumstance about a book 



