220 OPERATIONS SUBSIDIARY TO INDUCTION. 



not be those which serve hest 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 inference. 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 defini- 

 tion. It is impossible 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 on the use of words but on the properties of things, 

 Dr. Whewell calls the Explication of Conceptions. The act 

 of ascertaining, better than before, in what particulars any 

 phenomena which are classed together agree, he calls in his 

 technical phraseology, unfolding the general conception in 

 virtue of which they are so classed. Making allowance for 

 what appears to me the darkening and misleading tendency of 

 this mode of expression, several of his remarks are so much 

 to the purpose, that I shall take the liberty of transcribing 

 them. 



He observes,* that many of the controversies which have 

 had an important share in the formation of the existing body 

 of science, have "assumed the form of a battle of Definitions. 

 For example, the inquiry concerning the laws of falling bodies, 

 led to the question whether the proper definition of a uniform 

 force is that it generates a velocity proportional to the space 

 from rest, or to the time. The controversy of the vis viva was 

 what was the proper definition of the measure of force. A 



* Novum Oryanum Renovatum, pp. 35-37. 



