154 GRAVITATION— THE EXPLANATION 



a unified description, just as we have welded together 

 results of exploration with a scale and a clock. Some 

 progress has been made towards this unification. There 

 is, however, a real reason for admitting a partially 

 separate treatment; the one mode of exploration deter- 

 mines the symmetrical properties and the other the 

 antisymmetrical properties of the underlying world- 

 structure.* 



Objection has often been taken, especially by philo- 

 sophical writers, to the crudeness of Einstein's initial 

 requisitions, viz. a clock and a measuring scale. But the 

 body of experimental knowledge of the world which 

 Einstein's theory seeks to set in order has not come into 

 our minds as a heaven-sent inspiration; it is the result 

 of a survey in which the clock and the scale have actually 

 played the leading part. They may seem very gross 

 instruments to those accustomed to the conceptions of 

 atoms and electrons, but it is correspondingly gross 

 knowledge that we have been discussing in the chapters 

 concerned with Einstein's theory. As the relativity 

 theory develops, it is generally found desirable to replace 

 the clock and scale by the moving particle and light- 

 ray as the primary surveying appliances; these are test 

 bodies of simpler structure. But they are still gross 

 compared with atomic phenomena. The light-ray, for 

 instance, is not applicable to measurements so refined 

 that the diffraction of light must be taken into account. 

 Our knowledge of the external world cannot be divorced 

 from the nature of the appliances with which we have 

 obtained the knowledge. The truth of the law of gravi- 

 tation cannot be regarded as subsisting apart from the 

 experimental procedure by which we have ascertained 

 its truth. 



* See p. 236. 



