DEFINITION. 157 



change more or less extensive in the particulars 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 a special or technical defini- 

 tion 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 defini- 

 tion to expound the ordinary, and what they deemed the 

 natural, classification of things, namely, the division of them 

 into Kinds ; and to show the place which each Kind occupies, 

 as superior, collateral, or subordinate, among other Kinds. 

 This notion would account for the rule that all definition 

 must necessarily be per genus et differ entiam, and would also 

 explain why a single differentia was deemed sufficient. But 

 to expound, or express in words, a distinction of Kind, has 

 already been shown to be an impossibility : the very meaning 

 of a Kind is, that the properties which distinguish it do not 

 grow out of one another, and cannot therefore be set forth in 

 words, even by implication, otherwise than by enumerating 

 them all : and all are not known, nor are ever likely to be so. 

 It is idle, therefore, to look to this as one of the purposes of a 

 definition : while, if it be only required that the definition of a 

 Kind should indicate what Kinds include it or are included by 

 it, any definitions which expound the connotation of the names 

 will do this : for the name of each class must necessarily con- 

 note enough of its properties to fix the boundaries of the class. 

 If the definition, therefore, be a full statement of the connota- 

 tion, it is all that a definition can be required to be. 



5. Of the two incomplete and popular modes of defini- 

 tion, and in what they differ from the complete or philoso- 

 phical mode, enough has now been said. We shall next examine 

 an ancient doctrine, once generally prevalent and still by no 

 means exploded, which I regard as the source of a great part 

 of the obscurity hanging over some of the most important 

 processes of the understanding in the pursuit of truth. 

 According to this, the definitions of which we have now 



