342 INTRODUCTION TO PHILOSOPHY OF SCIENCE 



relations." x An alternative term is "particle," which has 

 already been employed earlier in the chapter. A particle 

 may then be defined as that which exhibits certain measur- 

 able spatial, temporal, and force properties. A particle may 

 be defined by approximately the following postulates : 



1. Every particle exhibits location in time. This is simply 

 the refinement of the corresponding empirical property of 

 things. It asserts that a particle occurs in the temporal 

 stream, though it does not state that the particle endures. 



2. Every particle exhibits location in space. This, again, 

 asserts not extension but mere location. Spatial location is 

 considered one of the basic properties of particles. 



3. Every particle is at some point or other at all instants. 

 This expresses the enduring feature of particles, and justifies 

 the principle of the indestructibility of matter. It demands 

 that every particle have a certain recognizable identity 

 through its history ; this persistent property is, as will be seen 

 immediately, its mass. Whatever changes in space or time a 

 particle undergoes, its mass remains constant (subject to 

 certain corrections due to the general theory of relativity). 



4. No particle can be at two different points at the same in- 

 stant. This states definitely that a particle is not extended, a 

 refinement which is introduced by science to avoid complica- 

 tions of size and shape. In many mechanical transactions a 

 thing acts as though it were located at a point, hence varia- 

 tions of extent and shape are of no significance. 



From these postulates the property of motion may be 

 deduced as a corollary. If a particle occupies and endures 

 through time, and if it occupies one but not more than one 

 point in space at an instant, it is capable of motion and rest 

 as these notions were defined earlier in the chapter. 



5. No two particles can be at the same point at the same 

 instant. This is the justification for the principle of the 

 impenetrability of matter. 



6. Every particle exhibits mass. This is the scientific sub- 

 stitute for the vague empirical notion of force. It is de- 



1 Physical Theory, p. 19, 



