402 THE PRINCIPLES OF SCIENCE. 



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

 publication, size, the initial words of the book itself, 

 of the title-page, the 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 arrange- 

 ment may be useful, for. we may happen to remember one 

 circumstance about a book when we have forgotten all 

 others ; but as we cannot usually go to the expense of 

 forming more than two or three indices at the most, we 

 must of course select those circumstances for the basis 

 of arrangement which will be likely to lead to the dis- 

 covery of a book most surely. Many of the criteria 

 mentioned are evidently inapplicable. The language in 

 which a book is written is no doubt definite enough, but 

 would afford no criterion for the classification of any large 

 group of English books, or of those written in any one 

 language. Classification by subjects would be an exceed- 

 ingly useful method if it were practicable, but experience, 

 or indeed a little reflection, shows it to be a logical 

 absurdity. It is a very difficult matter to classify the 

 sciences, so close and complicated are in many cases the 

 relations between them. But with books the complica- 

 tion is infinitely greater, since the same book may treat 

 successively of different sciences, or it may discuss a 

 problem involving many entirely diverse principles and 

 branches of knowledge. A good history of the steam 

 engine will be antiquarian, so far as it traces out records 

 of the earliest efforts at discovery ; purely scientific, as 

 regards the principles of thermodynamics involved ; 

 technical, as regards the mechanical means of applying 

 those principles ; economical, as regards the industrial 

 results of the invention ; biographical, as regards the 

 lives of the inventors. A history of Westminster Abbey 

 might belong either to the history of architecture, the 

 history of the church, or the history of England. If we 



