110 NAMES AND PEOPOSITIONS. 



gen, caused the second property, composition from a base and oxygen, to 

 be excluded from the connotation. The same discovery fixed the attention 

 of chemists upon hydrogen as an important element in acids; and more 

 recent discoveries having led to the recognition of its presence in sulphuric, 

 nitric, and many other acids, where its existence was not previously sus- 

 pected, there is now a tendency to include the presence of this element in 

 the connotation of the word. But carbonic acid, silica, sulphurous acid, 

 have no hydrogen in their composition ; that property can not, therefore, 

 be connoted by the term, unless those substances are no longer to be con- 

 sidered acids. Causticity and fluidity have long since been excluded from 

 the characteristics of the class, by the inclusion of silica and many other 

 substances in it ; and the formation of neutral bodies by combination with 

 alkalis, together with such electro-chemical peculiarities as this is supposed 

 to imply, are now the only differentiiK which form the fixed connotation of 

 the word Acid, as a term of chemical science. 



What is true of the definition of any term of science, is of course true of 

 the definition of a science itself; and accordingly (as observed in the In- 

 troductory Chapter of this work), the definition of a science must neces- 

 sarily be progressive and provisional. Any extension of knowledge or al- 

 teration in the current opinions respecting the subject-matter, may lead to 

 a 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 defin- 

 ing its name. 



In the same manner in which 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 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 germs et differentiam, and would 

 also explain why a single differentia was deemed sufiicient. But to ex- 

 pound, 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 can not 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 connota- 

 tion of the names will do this : for the name of each class must necessarily 

 connote enough of its properties to fix the boundaries of the class. If the 

 definition, therefore, be a full statement of the connotation, it is all that a 

 definition can be required to be.* 



* Professor Bain, in his Logic, takes a peculiar view of Definition. He holds (i., 71) with 

 the present work, that "the definition in its full import, is the sum of all the properties con- 

 noted by the name ; it exhausts the meaning of a word." But he regards the meaning of a 

 general name as including, not indeed all the common properties of the class named, but all 

 of them that are ultimate properties, not resolvable into one another. "The enumeration of 

 the attributes of oxygen, of gold, of man, should be an enumeration of the final (so fiir as can 

 be made out), the underivable, powers or functions of each," and nothing less than this is a 

 complete Definition (i., 75). An independent property, not derivable from other properties, 

 even if previously unknown, yet as soon as discovered becomes, according to him, part of the 



