NATURE'S PLAN OF STRUCTURE 27 



merely recognising that the pound is relative and there- 

 fore must not be expected to have those properties that 

 we had attributed to it in the belief that it was absolute. 

 You can form some idea of the essential difference in 

 the outlook of physics before and after Einstein's 

 principle of relativity by comparing it with the difference 

 in economic theory which comes from recognising the 

 •relativity of value of money. I suppose that in stable 

 times the practical consequences of this relativity are 

 manifested chiefly in the minute fluctuations of foreign 

 exchanges, which may be compared with the minute 

 changes of length affecting delicate experiments like the 

 Michelson-Morley experiment. Occasionally the con- 

 sequences may be more sensational — a mark-exchange 

 soaring to billions, a high-speed 8 particle contracting 

 to a third of its radius. But it is not these casual mani- 

 festations which are the main outcome. Clearly an 

 economist who believes in the absoluteness of the pound 

 has not grasped the rudiments of his subject. Similarly 

 if we have conceived the physical world as intrinsically 

 constituted out of those distances, forces and masses 

 which are now seen to have reference only to our own 

 special reference frame, we are far from a proper under- 

 standing of the nature of things. 



Nature's Plan of Structure. Let us now return to the 

 observer who was so anxious to pick out a "right" 

 frame of space. I suppose that what he had in mind 

 was to find Nature's own frame — the frame on which 

 Nature based her calculations when she poised the 

 planets under the law of gravity, or the reckoning of 

 symmetry which she used when she turned the electrons 

 on her lathe. But Nature has been too subtle for him; 

 she has not left anything to betray the frame which she 



