242 OPERATIONS SUBSIDIARY TO INDUCTION. 



numerous. Our choice is first limited by the pre- 

 ference to be given to properties which are well 

 known, and familiarly predicated of the class; but 

 even these are often too numerous to be all included 

 in the definition, and, besides, the properties most 

 generally known may not be those which serve best 

 to mark out the class from all others. We should 

 therefore select from among the common properties 

 (if among them any such are to be found), those 

 on which it has been ascertained by experience, or 

 proved by deduction, that many others depend ; or 

 at least which are sure marks of them, and from 

 whence, therefore, many others will follow by in- 

 ference. We thus see that to frame a good definition 

 of a name already in use, is not a matter of choice but 

 of discussion, and discussion not merely respecting 

 the usage of language, but respecting the properties 

 of things, and even the origin of those properties. 

 And hence every enlargement of our knowledge of 

 the objects to which the name is applied, is liable to 

 suggest an improvement in the definition. It is impos- 

 sible to frame a perfect set of definitions on any subject, 

 until the theory of the subject is perfect : and as science 

 makes progress, its definitions are also progressive. 



4. The discussion of Definitions, in so far as it 

 does not turn upon the use of words but upon the 

 properties of things, Mr. Whewell calls the Explica- 

 tion of Conceptions. The act of ascertaining, better 

 than before, in what particulars any phenomena which 

 are classed together agree, Mr. Whewell in his tech- 

 nical phraseology calls, unfolding the general concep- 

 tion in virtue of which they are so classed. Making 

 allowance for what appears to me the darkening and 



