100 REPLIES TO CRITICISMS. 



respecting planetary orbits and planetary motions, yet it was 

 like the successful among tliese in its relation to astronomical 

 phenomena : it was one of many possible hypotheses, which 

 admitted of having their consequences worked out and 

 tested ; and one which, on having its implications compared 

 with the results of observation, was found to explain them. 

 In short, the theory of gravitation grew out of experiences 

 of terrestrial phenomena; but the verification of it was 

 reached through experiences of celestial phenomena. Pass 

 ing now from premiss to inference, I do not see that, even 

 were the alleged parentage substantiated, it would necessitate 

 the supposed inseparability ; any more than the descent of 

 Geometry from land-measuring necessitates a persistent union 

 of the two. In the case of Algebra, as above indicated, 

 the disclosed laws of quantitative relations hold through 

 out multitudinous orders of phenomena that are extremely 

 heterogeneous ; and this makes conspicuous the distinction 

 between the theory and its applications. Here the laws of 

 quantitative relations among masses, distances, velocities, and 

 momenta, being applied mainly (though not exclusively) to 

 the concrete cases presented by Astronomy, the distinction 

 between the theory and its applications is less conspicuous. 

 But, intrinsically, it is as great in the one case as in the 

 other. 



How great it is, we shall see on taking an analogy. This 

 is a living man, of whom we may know little more than that 

 he is a visible, tangible person ; or of whom we may know 

 enough to form a voluminous biography. Again, this book 

 tells of a fictitious hero, who, like the heroes of old romance, 

 may be an impersonated virtue or vice, or, like a modern 

 hero, one of mixed nature, whose various motives and con 

 sequent actions are elaborated into a semblance of reality. 

 But no accuracy and completeness of the picture makes this 

 fictitious personage an actual personage, or brings him any 

 nearer to one. Nor does any meagreness in our knowledge 



