262 POINTER READINGS 



aspects of the world, whereas we are here taking the 

 macroscopic outlook. Confining ourselves to mechanics, 

 which is the subject in which the law of gravitation 

 arises, matter may be defined as the embodiment of three 

 related physical quantities, mass (or energy), momentum 

 and stress. What are "mass", "momentum" and 

 "stress"? It is one of the most far-reaching achieve- 

 ments of Einstein's theory that it has given an exact 

 answer to this question. They are rather formidable 

 looking expressions containing the potentials and their 

 first and second derivatives with respect to the co- 

 ordinates. What are the potentials? Why, that is just 

 what I have been explaining to you! 



The definitions of physics proceed according to the 

 method immortalised in "The House that Jack built" : 

 This is the potential, that was derived from the interval, 

 that was measured by the scale, that was made from the 

 matter, that embodied the stress, that. . . . But instead 

 of finishing with Jack, whom of course every youngster 

 must know without need for an introduction, we make 

 a circuit back to the beginning of the rhyme: . . . that 

 worried the cat, that killed the rat, that ate the malt, 

 that lay in the house, that was built by the priest all 

 shaven and shorn, that married the man. . . . Now we 

 can go round and round for ever. 



But perhaps you have already cut short my explana- 

 tion of gravitation. When we reached matter you had 

 had enough of it. "Please do not explain any more, 

 I happen to know what matter is." Very well; matter 

 is something that Mr. X knows. Let us see how it goes : 

 This is the potential that was derived from the interval 

 that was measured by the scale that was made from the 

 matter that Mr. X knows. Next question, What is Mr. X? 



Well, it happens that physics is not at all anxious to 



