MEASUREMENTS I4I 



which alone we could reach absolute time, of any body in 

 our perceptual experience. 



^Astronomy says it is not in me, nor do we get a more 

 definite answer from physics. Suppose an observer to 

 measure the distance traversed by light in one second? Is 

 this at all times a fixed standard of the length of a second? 

 A thousand years later another observer again measures the 

 distance for one of his seconds and finds it differs from the 

 old determination. What shall he infer? Is the speed of 

 light really variable, has the planetary system reached a 

 denser portion of the ether, has the second changed its 

 value, or does the fault lie with one or other observer? No 

 more than the astronomer can the physicist provide us 

 with an absolute measure of time. So soon as we grasp 

 this, we appear to lose our hold on time. The earth, the 

 sole clock by which we can measure milUons of years, fails 

 us when we once doubt its regularity. Why should a year 

 now represent the same amount of consciousness as it 

 might have done a few million years back? The absolutely 

 uniform motion by which alone we could reach an absolute 

 measurement of time, fails us in perceptual experience. It 

 is, like the geometrical surface, reached in conception and 

 in conception only, by carrying to a limit there the approxi- 

 mate sameness and uniformity which we observe in certain 

 perceptual movements.' 



Newton, when defining what Pearson will later on call 

 ^conceptual time', thus expresses himself: 



'That absolute, true, and mathematical time is conceived 

 as flowing at a constant rate, unaffected by the speed or 

 slowness of the motions of material things.' 



Clearly, such a time is a pure ideal, for how can we measure 

 it if there be nothing in the sphere of our perception which 

 we are certain flows at a constant rate? 'Uniform flow', like 

 any other scientific concept, is an ideal limit drawn from the 



