KEQUISITES OF LANGUAGE. 471 



the proper definition of the measure offeree. A principal question in the 

 classification of minerals is, what is the definition of a mineral species. 

 Physiologists have endeavored to throw light on their subject by defining 

 organization, or some similar term." Questions of the same nature were 

 long open and are not yet completely closed, respecting the definitions of 

 Specific Heat, Latent Heat, Chemical Combination, and Solution, 



" It is very important for us to observe, that these controversies have 

 never been questions of insulated and arbitrary definitions, as men seem 

 often tempted to imagine them to have been. In all cases there is a tacit 

 assumption of some proposition which is to be expressed by means of the 

 definition, and which gives it its importance. The dispute concerning the 

 definition thus acquires a real value, and becomes a question concerning 

 true and false. Thus, in the discussion of the question. What is a uniform 

 force? it was taken for granted that gravity is a uniform force. In the 

 debate of the vis viva, it was assumed that in the mutual action of bodies 

 the whole effect of the force is unchanged. In the zoological definition of 

 species (that it consists of individuals which have, or may have, sprung 

 from the same parents), it is presumed that individuals so related resemble 

 each other more^ than those which are excluded by such a definition ; or, 

 perhaps, that species so defined have permanent and definite differences. 

 A definition of organization, or of some other term which was not employed 

 to express some principle, would be of no value. 



"The establishment, therefore, of a right definition of a term, may be a 

 useful step in the explication of our conceptions; but this wall be the case 

 then only when we have under our consideration some proposition in which 

 the term is employed. For then the question really is, how the conception 

 shall be understood and defined in order that the proposition may be true. 



" To unfold our conceptions by means of definitions has never been serv- 

 iceable to science, except when it has been associated with an immediate 

 use of the definitions. The endeavor to define a Uniform Force was com- 

 bined with the assertion that gravity is a uniform force; the attempt to 

 define Accelerating Force was immediately followed by the doctrine that 

 accelerating forces may be compounded; the process of defining Momen- 

 tum was connected with the principle that momenta gained and lost are 

 equal ; naturalists would have given in vain the definition of Species which 

 we have quoted, if they had not also given the characters of species so sepa- 



i*ated Definition may be the best mode of explaining our conception, 



but that which alone makes it worth while to explain it in any mode, is the 

 opportunity of using it in the expression of truth. When a definition is 

 propounded to us as a useful step in knowledge, we are always entitled to 

 ask what principle it serves to enunciate," 



In giving, then, an exact connotation to the phrase, "a uniform force," 

 the condition was understood, that the phrase should continue to denote 

 gravity. The discussion, therefore, respecting the definition, resolved itself 

 into this question. What is there of a uniform nature in the motions pro- 

 duced by gravity ? By observations and comparisons, it was found that 

 what was uniform in those motions was the ratio of the velocity acquired 

 to the time elapsed ; equal velocities being added in equal times. A uni- 

 form force, therefore, was defined a force which adds equal velocities in 

 equal times. So, again, in defining momentum. It was already a received 

 doctrine that, when two objects impinge upon one another, the momentum 

 lost by the one is equal to that gained by the other. This proposition it 

 was deemed necessary to preserve, not from the motive (which operates in 



