LIMITS OF PHYSICAL KNOWLEDGE 257 



less in physics. We use instead the law of conserva- 

 tion of mass (either as an empirical law or deduced 

 from the law of gravitation) which assures us that, 

 provided the tube is isolated, the pointer reading on 

 the schedule derived from the weighing-machine type 

 of experiment has a constant value along the tube. 

 For the purpose of exact science "the same object" 

 becomes replaced by "isolated world-tube". The con- 

 stancy of certain properties of the elephant is not 

 assumed as self-evident from its sameness, but is an 

 inference from experimental and theoretical laws re- 

 lating to world-tubes which are accepted as well 

 established. 



Limitations of Physical Knowledge, Whenever we state 

 the properties of a body in terms of physical quantities 

 we are imparting knowledge as to the response of 

 various metrical indicators to its presence, and nothing 

 more. After all, knowledge of this kind is fairly com- 

 prehensive. A knowledge of the response of all kinds 

 of objects — weighing-machines and other indicators — 

 would determine completely its relation to its environ- 

 ment, leaving only its inner un-get-atable nature un- 

 determined. In the relativity theory we accept this as 

 full knowledge, the nature of an object in so far as it is 

 ascertainable by scientific inquiry being the abstraction 

 of its relations to all surrounding objects. The progress 

 of the relativity theory has been largely due to the 

 development of a powerful mathematical calculus for 

 dealing compendiously with an infinite scheme of 

 pointer readings, and the technical term tensor used so 

 largely in treatises on Einstein's theory may be translated 

 schedule of pointer readings. It is part of the aesthetic 

 appeal of the mathematical theory of relativity that the 



