REQUISITES OF LANGUAGE. 223 



-which a hody, when it sets another body in motion, loses 

 exactly as much as it communicates ? And when experiment 

 had shown that this something was the product of the velocity 

 of the body by its mass, or quantity of matter, this became 

 the definition of momentum. 



The following remarks,* therefore, are perfectly just: 

 " The business of definition is part of the business of dis- 

 covery To define, so that our definition shall have 



any scientific 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 Definition 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 specu- 

 lative views." For in order to judge finally 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 great part in producing that general 

 air of resemblance which was the original inducement to the 

 formation of the class, the definition will then be most feli- 

 citous. 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, founded his definition of life on the process of decom- 

 position and recomposition which incessantly takes place 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 great 

 authorities (independently of M. de Blainville, who is himself 



* Nov. Org. Renov., pp. 39, 40. 



