

MOTION, FORCE, MATTER 317 



motion: scientific content 



There is not one concept of motion which functions in 

 science, but two. Corresponding to the Newtonian-Euclidean 

 absolute space and time there is an absolute motion, and 

 corresponding to the space-time of relativity there is a 

 relative motion. For purposes of illustration emphasis will 

 be placed on the former, with concluding remarks showing 

 the changes that must be introduced when the facts of 

 relativity are taken into consideration. 



Dependence on particles. The abstractive operation in- 

 volved in the passage from empirical motion to scientific 

 motion is not so great as that involved in the passage from 

 empirical space and time to absolute space and time. Abso- 

 lute motion cannot be empty; it is defined as "the occupa- 

 tion, by one entity, of a continuous series of places at a con- 

 tinuous series of times." l Abstractly, any correlation of the 

 spatial continuum with the temporal continuum is possible; 

 but only certain of these correlations are motions. The cor- 

 relation desired is accomplished through the notion of the 

 particle. This is merely a refinement of the notion of event, 

 or object, which is demanded in the empirical conception of 

 motion. For science, as for common sense, there is always 

 something which moves. For the purposes of defining motion 

 a particle may be considered to be any entity which can be 

 at a point in space at an instant in time. The more precise 

 characterization of this notion will be given later in the 

 chapter. For the present, however, one may say that a par- 

 ticle is any entity which through the notions of "being at a 

 point," and "being at an instant" brings about the correla- 

 tion between space and time by which motion is to be de- 

 fined. Absolute motion, therefore, is dependent on the con- 

 cept of the particle. 



Dependence on measurement. Motion as it functions in 

 science is more closely tied up with activities of measure- 

 ment than is empirical motion. Empirical motion is a given 



1 Bertrand Russell, Principles of Mathematics, p. 469. 



