246 OPERATIONS SUBSIDIARY TO INDUCTION. 



Mr. Whewell very justly adds*, u The business 

 of definition is part of the business of discovery. . . . 

 To define, so that our definition shall have any sci- 

 entific value, requires no small portion of that sagacity 

 by which truth is detected. . . When it has been 

 clearly seen what ought to be our definition, it must 

 be pretty well known what truth we have to state. 

 The definition, as well as the discovery, supposes a 

 decided step in our knowledge to have been made. 

 The writers on Logic, in the middle ages, made Defi- 

 nition the last stage in the progress of knowledge; 

 and in this arrangement at least, the history of 

 science, and the philosophy derived from the history, 

 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 

 dependance 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 great part 

 in producing that general and superficial air of resem- 

 blance which was the original inducement to the 

 formation of the class, the definition will then be 

 most felicitous. But it is often necessary to define 

 the class by some property not familiarly known, 

 provided 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 pro- 



Phil. of the Ind. Sc. t ii., 181-2. 



