On Definitions. 267 



Now we shall find, by a careful consideration of the subject, that 

 there is only one way in which this verbal description can be obtain- 

 ed ; only one way in which a strictly logical definition can be formed. 

 I say, strictly logical, because there are innumerable explanations to 

 be found in dictionaries and other books, which are not definitions, 

 but for the most part mere illustrations, serving by means of syno- 

 nyms, and other analogous words, to produce a vague and uncertain 

 approximation to the meaning intended. What then is the way by 

 which alone this purpose can be accomplished ? It is by stating the 

 genus, or higher class, to which it belongs, and then the differences 

 which distinguish it from other species, or from other objects. It 

 must specify the genus, and the proper difference. If either one or 

 more of these terms is not already perfectly understood, the defini- 

 tion is of no use. 



Thus, if I would define the word Iron, I must state that it is a 

 metal, black, of a certain specific gravity, malleable, admitting a fine 

 polish, and becoming liquid at a certain temperature, &:c. Here, 

 metal is the genus, or higher class, and the other qualities that fol- 

 low, form the characteristic differences, which distinguish this from 

 other metals. But, if either the word Metal, which expresses the 

 genus, or any of the qualities next mentioned, are unknown to the 

 person we seek to inform, we must make up that deficiency, or lay 

 aside the definition. But if I would define a metal, I must state, 

 that it is a solid, and then mention all the properties of metals. 



If I am required to define the word Square, I may call it a rect- 

 angle, having two adjacent sides equal. Here, a rectangle is the 

 genus, having two adjacent sides equal, the characteristic difference. 

 If either term is unknown, the definition is useless, or rather not one. 

 I might also define it, a four-sided figure, having two sides parallel 

 and one right angle. 



A definition is also deficient, or not a proper definition, if either 

 the genus does not comprehend the species or individual intended ; 

 or, if the enumeration of differences be incomplete, so as not to dis- 

 tinguish it from all the other species or individuals comprehended 

 under the genus mentioned. It would be redundant, if any of the 

 differences were implied in the term denoting the genus ; or, if any 

 of the differences comprehended another difference mentioned. 



In all the sciences, before attempting to lay down definitions, it is 

 necessary that the meaning of certain terms and expressions, be pre- 

 viously settled by agreement, according to the methods already ex- 



