THE UNFATHOMED UNIVERSE 11 



gravitation holds absolutely, and states without hypothesis 

 how the motions of the bodies and the earth are mutually 

 affected. But if we proceed, with Le Sage or some other, 

 to a theory of gravitation, we enter a realm of imaginative 

 construction. We work with concepts which are, no doubt, 

 ingenious, well-thought-out, consistent, and useful ; but how 

 far they correspond to, or are representative of objective 

 entities is a question to be carefully considered. Sometimes 

 the concepts that are effectively worked with are obviously 

 mere symbols; thus no one supposes that carbon atoms are 

 really like their quite useful quadrumanous diagrammatic 

 representations in books on chemistry. The difficulty is in 

 regard to subtler symbols or concepts, which work so well 

 that we inevitably come to think of them as objective actual- 

 ities. And it may be that what was at first an imaginative 

 thought-economising symbol, part of a system of intellectual 

 shorthand, will be actually verified in Nature. Thus many 

 physicists now speak confidently and convincingly of the 

 ' reality ' of the atom. When it is legitimate to speak of 

 a scientific symbol as real is a matter for the experts only, 

 as is plain enough from their disagreement. Thus the ether 

 was declared by Lord Kelvin to be the greatest certainty in 

 physics, while we find the upholders of the Principle of 

 Relativity declaring that the Victorian ether and the Vic- 

 torian matter must both go. Similarly in biological prob- 

 lems, such as those centred in inheritance, there is diver- 

 gence of opinion as to the objectivity of ' biophores ', 

 1 determinants ', ' factors ', and ' genes ', though these are 

 very useful in formulating conclusions and prompting fur- 

 ther questions. 



(c) Third, it has to be remembered that the descriptive 

 formulae are more than summations of the routine of in- 



