472 OPERATIONS SUBSIDIARY TO INDUCTION. 



many other cases) that it was firmly fixed in popular belief; for the propo- 

 sition in question had never been heard of by any but the scientifically in- 

 structed. But it was felt to contain a truth ; even a superficial observation 

 of the phenomena left no doubt that in the propagation of motion from 

 one body to another, there was something of which the one body gained 

 precisely what the other lost ; and the word momentum had been invented 

 to express this unknown something. The settlement, therefore, of the defi- 

 nition of momentum, involved the determination of the question. What is 

 that of which a body, 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 quan- 

 tity of mattei*, this became the definition of momentum. 



The following remarks,* therefore, are perfectly just: "The business of 



definition is part of the business of discovery 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 de- 

 cided step in our knowledge to have been made. The Av^'iters on Logic, 

 in the Middle Ages, made Definition the last stage in the progress of knowl- 

 edge ; and in this arrangement at least, the history of science, and the phi- 

 losophy dei'ived from the history, confirm their speculative 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 com- 

 mon 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 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, founded his defini- 

 tion of life on the process of decomposition and recomposition which in- 

 cessantly 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 a first-rate authority) have thought that no 

 other property so well answers the conditions I'equired 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 princi- 

 ples, and that even when practicable, it is occasionally not desirable. 



Cases in which it is impossible to comply M'ith all the conditions of a 

 precise definition of a name in agreement with usage, occur very frequent- 

 ly. There is often no one connotation capable of being given to a word, 

 so that it shall still denote every thing it is accustomed to denote ; or that 

 all the propositions into which it is accustomed to enter, and Avhich have 

 any foundation in truth, shall remain true. Independently of accidental 

 ambiguities, in which the different meanings have no connection with one 



* Novum Organum Renovaium, pp. 39, 40. 



