DEFINITION. 193 



and one of those of which the popular sense is plainest 

 and best understood. The word I mean is Heat ; and 

 the source of the difficulty is the imperfect state of 

 our scientific knowledge, which has shown to us mul- 

 titudes of phenomena certainly connected with the 

 same power which is the cause of what our senses 

 recognise as heat, but has not yet taught us the laws 

 of those phenomena with sufficient accuracy to admit 

 of our determining, under what characteristics the 

 whole of those phenomena shall ultimately be embo- 

 died as a class : which characteristics would of course 

 be so many differentiae for the definition of the power 

 itself. We have advanced far enough to know that 

 one of the attributes connoted must be that of ope- 

 rating as a repulsive force : but this is certainly not 

 all which must ultimately be included in the scientific 

 definition of heat. 



What is true of the definition of any term of 

 science, is of course true of the definition of a science 

 itself: and accordingly, we showed in the Introduc- 

 tory Chapter of this work, that the definition of a 

 science must necessarily be progressive and provisional. 

 Any extension of knowledge, or alteration in the 

 current opinions respecting the subject matter, may 

 lead to a change more or less extensive in the parti- 

 culars included in the science; and its composition 

 being thus altered, it may easily happen that a different 

 set of characteristics will be found better adapted as 

 differentiae for defining its name. 



In the same manner in which, as we have now 

 shown, a special or technical definition has for its 

 object to expound the artificial classification out of 

 which it grows ; the Aristotelian logicians seem to have 

 imagined that it was also the business of ordinary 

 definition to expound the ordinary, and what they 

 VOL. i. o 



