INTRODUCTION. 25 



such, great value to the modern methods of mechanical 

 construction, according to which all the parts of a 

 machine are exact facsimiles of a fixed pattern. All 

 the rifles used, in the British army are constructed on 

 the interchangeable system, so that any one part of any 

 one rifle can be substituted indifferently for the same 

 part of another. A bullet fitting one rifle will fit all 

 others of the same bore. Sir J. Whitworth has extended 

 the same system to the screws and screw-bolts used in 

 connecting together the parts of machines, by establishing 

 a series of standard screws. 



Anticipations of the Principle of Substitution. 



In such a subject as logic it is hardly possible to put 

 forth any opinions or principles which have not been 

 in some degree previously entertained. The germ at 

 least of every doctrine will be found in earlier writers, 

 and novelty must arise chiefly in the mode of harmonising 

 and developing ideas. When I first proposed to employ 

 the process and name of substitution in logic r , I believe 

 that I was led to do so from analogy with the familiar 

 mathematical process of substituting for a symbol its value 

 as given in an equation. In writing- my first logical essay 

 I had a most imperfect conception of the importance 

 and generality of the process, and I described, as if they 

 were of equal importance, a number of other laws which 

 now seem to be but particular cases of the one general 

 rule of substitution. 



My second essay, the Substitution of Similars, was 

 written shortly after I had become aware of the great 

 simplification which may be effected by a proper appli- 

 cation of the principle of substitution. I was not then 

 acquainted with the fact that the German logician 



r 'Pure Logic/ pp. 18-19. 



