102 DISCOURSE ON THE STUDY 



as to recognize a class possessing that of the exhi- 

 bition of periodical colours in polarized light, a mere 

 comparison of lists would at once demonstrate the 

 identity of the two classes, or enable us to ascertain 

 whether one was or was not included in the other. 



(94.) It is thus we perceive the high importance 

 in physical science of just and accurate classifica- 

 tions of particular facts, or individual objects, under 

 general well considered heads or points of agree- 

 ment (for which there are none better adapted 

 than the simple phenomena themselves into which 

 they can be analysed in the first instance) ; for by 

 so doing each of such phenomena, or heads of 

 classification, becomes not a particular but a general 

 fact ; and when we have amassed a great store of 

 such general facts, they become the objects of an- 

 other and higher species of classification, and are 

 themselves included in laws which, as they dis- 

 pose of groups, not individuals, have a far supe- 

 rior degree of generality, till at length, by con- 

 tinuing the process, we arrive at axioms of the 

 highest degree of generality of which science is 

 capable. 



(95.) This process is what we mean by induction ; 

 and, from what has been said, it appears that in- 

 duction may be carried on in two different ways, 

 either by the simple juxta-position and comparison 

 of ascertained classes, and marking their agree- 

 ments and disagreements ; or by considering the 

 individuals of a class, and casting about, as it were/ 

 to find in what particular they all agree, besides 

 that which serves as their principle of classification, 

 Either of these methods may be put in practice as 



