316 INTRODUCTION TO PHILOSOPHY OF SCIENCE 



motion as having taken place. Now it seems that motion on 

 the empirical level is properly described rather as something 

 taking place than as an inference to something which must 

 have taken place by virtue of the evidence afforded by cer- 

 tain data. " If a body is moving at all fast, we see its motion 

 just as we see its color. A slow motion, like that of the hour- 

 hand of a watch, is only known ... by observing a change 

 of position after a lapse of time; but, when we observe the 

 motion of the second-hand, we do not merely see first one 

 position and then another — we see something as directly 

 sensible as color." ! Motion, therefore, is an empirical datum, 

 and, moreover, one which resists analysis into anything 

 which is psychologically more primitive. There are motions 

 which are not reducible to a series of positions, but are per- 

 ceived as single facts. It is these which give the concept of 

 motion its empirical content. 



Quantity. To say that motion is empirically a quantity 

 is to characterize it in terms of a concept whose analysis 

 has already been made. Quantities are events which may be 

 ordered on the basis of the relation "greater than." Mo- 

 tions are, therefore, also velocities, and may be compared to 

 one another according to degree. Perceptual discernment en- 

 ables one to make crude judgments as to relative velocities, 

 e.g., that an express train travels faster than a horse-drawn 

 vehicle. In doubtful cases measurement is introduced. This 

 involves correlating the two motions either with the respec- 

 tive changes in space over a given time, or with the respective 

 times required to accomplish a given change in space. Fur- 

 thermore, motion, in addition to exhibiting variations in 

 intensity, manifests also variations in direction. Movement 

 of A and B toward one another is not the same as motion of 

 A and B away from one another, nor the same as motion of 

 A obliquely to a line joining A and B. Rotational and 

 "squirming" motions also exhibit features peculiar to them- 

 selves. These can be described as complicated functions of 

 changes in distance and direction. 



1 Bertrand Russell, Our Knowledge of the External World, p. 137. 



