MOTION, FORCE, MATTER 313 



and specific techniques, and hence that the meanings of the 

 concepts cannot be isolated from these physical events and 

 processes. Scientific motion, force, and matter are definable 

 as measured empirical motion, force, and matter but in that 

 one word lies a significant distinction which it will be the 

 task of this chapter to elucidate. 



motion: empirical foundation 



At the empirical level, motion may be described as a quan- 

 titatively variable change in the spatial relations holding be- 

 tween two events. The terms in this statement which require 

 clarification are "relation," "change," and "quantity." 



Relativity. Empirical motion is relative in two very obvi- 

 ous senses: It is relative to something which moves, and it is 

 relative to a point of reference. Consider the former of these 

 two senses. Motion is a property of objects or events; trains, 

 baseballs, clouds, raindrops, and planets move. Motion is 

 capable of analysis into (a) the motion itself, and (6) the 

 mover. What is given empirically, therefore, is not motion 

 as such but rather moving objects. All that seems to be 

 involved in this notion of objects is something which retains 

 a qualitative identity through the change. The notion 

 readily develops into the conception of an underlying "sub- 

 stance," existing beneath the observable qualities of the 

 object. There seems to be nothing, however, in the empirical 

 data that demands this hypothetical development. All that 

 is involved is the possibility of identifying the object which 

 moves throughout its history by comparing the object which 

 began the motion with the object which completed the 

 motion and noting enough similarity to permit them to be 

 designated as the same object. The conception of empirical 

 motion is closely allied to the conception of empirical space 

 and time. Just as there can be no empty space or time, for 

 space and time would then melt away, so there can be no 

 "empty' motion. Motion is not mere change of space 

 through a given duration, but change of something in space; 

 from the empirical point of view mere change of space is 



