THE LAWS OF MOTION 281 



becomes imperceptible), he deals with it as a moving 

 point. What right has the physicist to invent this ideal 

 particle ? He has never perceived the limiting quantity, 

 the minimum esse of a substance, and therefore cannot 

 assert that it would not produce in him sense-impressions 

 which could only be described by aid of the concepts spin 

 and strain. The logical right of the physicist is, however, 

 exactly that on which all scientific conceptions are based. 

 We have to ask whether postulating an ideal of this sort 

 enables us to construct out of the motion of groups of 

 particles those more complex motions by aid of which 

 we describe the physical universe. Is the particle a 

 symbol by aid of which we can describe our past and 

 predict our future sequences of sense-impressions with a 

 great and uniform degree of accuracy? If it be, then its 

 use is justified as a scientific method of simplifying our 

 ideas and of economising thought. 



The reader must note that this hypothesis of the 

 particle is made use of by Newton in the statement of 

 his law of gravitation : " Every particle of matter in the 

 universe attracts every other particle" he tells us, in such 

 and such a manner. Yet Newton is here dealing with 

 conceptual notions, for he never saw, nor has any physicist 

 since his time ever seen, individual particles, or been able 

 to examine how the motion of two such particles is related 

 to their position. The jusJ:ification of the law of gravitation 

 lies in the power it gives us of constructing the motion of 

 those groups of particles by aid of which we symbolise 

 physical bodies and ultimately describe and predict the 

 routine of our sense-impressions. The particle, therefore, 

 as the symbolic unit of physical substance with its simple 

 motion of translation is as valid as the law of gravitation, 

 in the statement of which it is indeed involved. 



Lastly, groups of particles bounded in conception by 

 continuous surfaces are the symbols by which we represent 

 those material groups of sense-impressions that are 

 currently spoken of as physical bodies or objects. To 

 find the simplest possible types of relative motion for 

 these various concepts, and thence to construct the motion 



