508 OPERATIONS SUBSIDIARY TO INDUCTION. 



The end of Classification, as an instrument for the investigation of nature, 

 is (as before stated) to make us think of those objects together which have 

 the greatest number of important common properties; and which, there- 

 fore, we have oftenest occasion, in the course of our inductions, for taking 

 into joint consideration. Our ideas of objects are thus brought into the 

 order most conducive to the successful prosecution of inductive inquiries 

 generally. But when the purpose is to facilitate some particular inductive 

 inquiry, more is required. To be instrumental to that purpose, the classifi- 

 cation must bring those objects together, the simultaneous contemplation 

 of which is likely to throw most light upon the particular subject. That 

 subject being the laws of some phenomenon or some set of connected phe- 

 nomena ; the very phenomenon or set of phenomena in question must be 

 chosen as the groundwork of the classification. 



The requisites of a classification intended to facilitate the study of a par- 

 ticular phenomenon, are, first to bring into one class all Kinds of things 

 which exhibit that phenomenon, in whatever variety of forms or degrees ; 

 and, secondly, to arrange those Kinds in a series according to the degree in 

 which they exhibit it, beginning with those Avhich exhibit most of it, and 

 terminating with those which exhibit least. The principal example, as yet, 

 of such a classification, is afforded by comparative anatomy and physiology, 

 from which, therefore, our illustrations shall be taken. 



§ 2. The object being supposed to be, the investigation of the laws of 

 animal life ; the first step, after forming the most distinct conception of 

 the phenomenon itself, possible in the existing state of our knowledge, is to 

 erect into one great class (that of animals) all the known Kinds of beings 

 where that phenomenon presents itself; in however various combinations 

 with other properties, and in however different degrees. As some of these 

 Kinds manifest the general phenomenon of animal life in a very high de- 

 gree, and others in an insignificant degree, barely sufticient for recognition ; 

 we must, in the next place, arrange the various Kinds in a series, following 

 one another according to the degrees in which they severally exhibit the 

 phenomenon ; beginning therefore with man, and ending with the most im- 

 perfect kinds of zoophytes. 



This is merely saying that we should put the instances, from which the 

 law is to be inductively collected, into the order which is implied in one of 

 the four Methods of Experimental Inquiry discussed in the preceding Book ; 

 the fourth Method, that of Concomitant Variations. As formerly remarked, 

 this is often the only method to which recourse can be had, with assurance 

 of a true conclusion, in cases in which we have but limited means of effecting, 

 by artificial experiments, a sepai-ation of circumstances usually conjoined. 

 The principle of the method is, that facts which increase or diminish to- 

 gether, and disappear together, are either cause and effect, or effects of a 

 common cause. When it has been ascertained that this relation really sub- 

 sists between the variations, a connection between the facts themselves may 

 be confidently laid down, either as a law of nature or only as an empirical 

 law, according to circumstances. 



That the application of this Method must be preceded by the formation 

 of such a series as we have described, is too obvious to need being pointed 

 out; and the mere arrangement of a set of objects in a series, according to 



It would surely be possible to arrange all places (for example) in the order of their distance 

 from the North Pole, though there would be not merely a plurality, but a whole circle of places 

 at every single gradation in the scale. 



