406 OPERATIONS SUBSIDIARY TO INDUCTION. 



the progress of knowledge ; and in this arrangement at least, the hi.H- 

 tory of science, and the philosophy derived from the histor}^ confirm 

 their speculative views." For in order to judge how the name which 

 denotes a class may best be defined, we must know all the properties 

 common to the class, and all the relations of causation or dependence 

 among those properties. 



If the properties which are fittest to be selected as marks of other 

 common properties are also obvious and familiar, and especially if they 

 bear a gj-eat part in producing that general and superficial air ofi re- 

 semblance which was the original inducement to the formation of the 

 class, the definition will then be most felicitous. But it is often neces- 

 sary to define the class by some property not familiarly known, provi- 

 ded that property be the best mark of those which are known. M. de 

 Blainville, for instance, has founded his definition of life, upon the 

 process of decomposition and recomposition which incessantly goes on 

 in every living body, so that the particles composing it are never for 

 two instants the same. This is by no means one of the most obvious 

 properties of living bodies; it might escape altogether the notice of an 

 unscientific observer. Yet gi-eat authorities (independently of M. de 

 Blainville, who is himself a first-rate authority,) have thought, seem- 

 ingly with much reason, that no other property so well answers the 

 conditions required for the definition. 



§ 5. Having laid down the principles which ought for the most part 

 to be observed in attempting to give a precise connotation to a term in 

 use, I must now add, that it is not always practicable to adhere to 

 those jjrinciples, and that even when practicable, it is occasionally not 

 desirable. Cases in which it is impossible to comply mth all the con- 

 ditions of a precise definition of a name in agreement with usage, occur 

 very frequently. There is often no one connotation capable of being 

 given to a word, so that it shall still denote everything it is accustomed 

 to denote ; or that all the propositions into which it is accustomed to 

 enter, and which have any foundation in truth, shall remain true. In- 

 dependently of accidental ambiguities, in which the different meanings 

 have no connexion with one another; it continually happens that a 

 word is used in two or more senses derived fi-om each other, but yet 

 radically distinct. So long as a term is vague, that is, so long as its 

 connotation is not ascertained and permanently fixed, it is constantly 

 liable to be applied by extension from one thing to another, until it 

 reaches things which have little, or even no, resemblance to those which 

 were first designated by it. ■ , 



Suppose, says Dugald Stewart, in his PhilmopJiical Essays ,*" that 

 the letters A, B, C, D, E, denote a series of objects ; that A possesses 

 some one quahty m common wdth B; B a quality in common with C; 

 C a quality in common witli D ; D a quality in common with E ; while 

 at the same time, no quality can be found which belongs in common 

 to any three objects in the series. Is it not conceivable, that the affin- 

 ity between A and B may produce a transference of the name of the 

 first to the second ; and that, in consequence of the other aflinities 

 which connect the remaining objects together, the same name m^' pas3 

 in succession from B to C ; from C to D ; and from D to E ? In this 



* P. 217, 4to edition. 



