314 INTRODUCTION TO PHILOSOPHY OF SCIENCE 



meaningless, for it represents a change in something which 

 has its form determined only by its content. What 

 moves is indifferent; but that something moves seems 

 essential. 



But, in the second place, motion is meaningless apart 

 from a point of reference. Motion is something which is 

 descriptive of a relation between events, and not of a single 

 event. As has already been seen, empirical space reduces to 

 position, and position reduces to relations of distance and 

 direction. Motion, therefore, which is change in position, 

 reduces to change in distance and direction — relations which 

 hold only between events or objects. On the empirical level 

 it is arbitrary whether one says that A moves and B remains 

 at rest, or B moves and A remains at rest. The air of paradox 

 which one finds in a concrete illustration of this, e.g., the 

 case of a train moving past a station, is dissipated when 

 one recalls two facts: (a) The scientific theory of absolute 

 motion has become so completely a part of the background 

 of the individual's experience that he tends to think of 

 motion in these terms rather than in terms of perceptual 

 data. The situation here is precisely what it was in connec- 

 tion with space and time. (6) Empirical motion is in some 

 way intimately associated with the feeling of effort which is 

 required to move objects; small objects are easier to move 

 than large objects, hence the former are more likely to move 

 than the latter. In the case of the train, the station is un- 

 consciously associated with all other objects with which it 

 has no relative motion and which have no motions relatively 

 to one another; the inherent difficulty involved in moving 

 so extensive a system suggests that it is at rest while only 

 the train is in motion. When the reference system is reduced 

 in extent and becomes, for example, another train on a 

 parallel track, the strict relativity of the motion again 

 emerges; it is a noteworthy fact that if an observer in the 

 latter case cannot see the ground and cannot feel the motion, 

 he is quite unable to decide whether his train or the one on 

 the parallel track is really moving. 



