i84 



INDUCTION. 



ordinary observation and practical 

 life (pp. 241-247). They must be 

 taken exclusively from the generalisa- 

 tions by which scientific thinkers have 

 ascended to great and comprehensive 

 laws of natural phenomena. Now it 

 is seldom possible, in these compli- 

 cated inquiries, to go much beyond 

 the initial steps without calling in 

 the instrument of Deduction and the 

 temporary aid of hypotheses ; as I 

 myself, in common with Dr. Whewell, 

 have maintained against the purely 

 empirical school. Since, therefore, 

 such cases could not conveniently be 

 selected to illustrate the principles of 

 mere observation and experiment. Dr. 

 Whewell is misled by their absence 

 into representing the Experimental 

 Methods as serving no purpose in 

 scientific investigation ; forgetting 

 that if those methods had not sup- 

 plied the first generalisations, there 

 would have been no materials for his 

 own conception of Induction to work 

 upon. 



His challenge, however, to point 

 out which of the four methods are 

 exemplified in certain important cases 

 of scientific inquiry is easily answered. 

 "The planetary paths," as far as they 

 are a case of induction at all,* fall 

 under the Method of Agreement. 

 The law of " falling bodies," namely, 

 that they describe spaces proportional 

 to the squares of the times, was his- 

 torically a deduction from the first 

 law of motion ; but the experiments 

 by which it was verified, and by which 

 it might have been discovei-ed, were 

 examples of the Method of Agree- 

 ment ; and the apparent variation 

 from the true law, caused by the re- 

 sistance of the air, was cleared up by 

 experiments in vacuo, constituting an 

 application of the Method of Differ- 

 ence. The law of " refracted rays " 

 (the constancy of the ratio between 

 the sines of incidence and of refrac- 

 tion for each refracting substance) 

 was ascertained by direct measure- 

 ment, and therefore by the Method 



* See, on this point, the second chapter 

 of the present Book. 



of Agreement. The "cosmical mo- 

 tions" were determined by highly 

 complex processes of thought, in which 

 Deduction was predominant, but the 

 Methods of Agreement and of Con- 

 comitant Variations had a large part 

 in establishing the empirical laws. 

 Every case without exception of 

 *' chemical analysis " constitutes a 

 well-marked example of the Method 

 of Difference. To any one acquainted 

 with the subjects — to Dr. Whewell 

 himself, there would not be the small- 

 est difficulty in setting out " the ABC 

 and a b c elements " of these cases. 



If discoveries are ever made by 

 observation and experiment without 

 Deduction, the four methods are 

 methods of discovery : but even if 

 they were not methods of discovery, 

 it would not be the less true that they 

 are the sole methods of Proof ; and 

 in that character, even the results of 

 deduction are amenable to them. The 

 great generalisations which begin as 

 Hypotheses must end by being proved, 

 and are in reality (as will be shown 

 hereafter) proved, by the Four Me- 

 thods. Now it is with Proof, as such, 

 that Logic is principally concerned. 

 This distinction has, indeed, no chance 

 of finding favour with Dr. Whewell ; 

 for it is the peculiarity of his system 

 not to recognise, in cases of Induc- 

 tion, any necessity for proof. If, after 

 assuming an hypothesis and carefully 

 collating it with facts, nothing is 

 brought to light inconsistent with it, 

 that is, if experience does not disprove 

 it, he is content ; at least until a sim- 

 pler hypothesis, equally consistent 

 with experience, presents itself. If 

 this be Induction, doubtless there is 

 no necessity for the Four Methods. 

 But to suppose that it is so appears 

 to me a radical misconception of the 

 nature of the evidence of physical 

 truths. 



So real and practical is the need of 

 a test for induction similar to the 

 syllogistic test of ratiocination, that 

 inferences which bid defiance to the 

 most elementary notions of inductive 

 logic are put forth without misgiv- 



