254 POINTER READINGS 



ings of a photometer for various wave-lengths of light. 

 And so on until all the characteristics of the elephant 

 are exhausted and it has become reduced to a schedule 

 of measures. There is always the triple correspond- 

 ence — 



(a) a mental image, which is in our minds and not in 

 the external world; 



(b) some kind of counterpart in the external world, 

 which is of inscrutable nature; 



(<:) a set of pointer readings, which exact science can 

 study and connect with other pointer readings. 



And so we have our schedule of pointer readings 

 ready to make the descent. And if you still think that 

 this substitution has taken away all reality from the 

 problem, I am not sorry that you should have a foretaste 

 of the difficulty in store for those who hold that exact 

 science is all-sufficient for the description of the universe 

 and that there is nothing in our experience which cannot 

 be brought within its scope. 



I should like to make it clear that the limitation of 

 the scope of physics to pointer readings and the like is 

 not a philosophical craze of my own but is essentially 

 the current scientific doctrine. It is the outcome of a 

 tendency discernible far back in the last century but 

 only formulated comprehensively with the advent of 

 the relativity theory. The vocabulary of the physicist 

 comprises a number of words such as length, angle, 

 velocity, force, potential, current, etc., which we call 

 "physical quantities". It is now recognised as essential 

 that these should be defined according to the way in 

 which we actually recognise them when confronted with 

 them, and not according to the metaphysical significance 

 which we may have anticipated for them. In the old 

 textbooks mass was defined as "quantity of matter"; 



