i6o GRAVITATION— THE EXPLANATION 



adopt any geometry we choose.* Naturally if we hold 

 ourselves free to apply any correction we like to our 

 experimental measures we can make them obey any 

 law; but was it worth while saying this? The asser- 

 tion that any kind of geometry is permissible could only 

 be made on the assumption that lengths have no fixed 

 value — that the physicist does not (or ought not to) 

 mean anything in particular when he talks of length. 

 I am afraid I shall have a difficulty in making my 

 meaning clear to those who start from the assumption 

 that my words mean nothing in particular; but for those 

 who will accord them some meaning I will try to remove 

 any possible doubt. The physicist is accustomed to state 

 lengths to a great number of significant figures; to 

 ascertain the significance of these lengths we must notice 

 how they are derived; and we find that they are derived 

 from a comparison with the extension of a standard of 

 specified material constitution. (We may pause to notice 

 that the extension of a standard material configuration 

 may rightly be regarded as one of the earliest subjects 

 of inquiry in a physical survey of our environment.) 

 These lengths are a gateway through which knowledge 

 of the world around us is sought. Whether or not they 

 will remain prominent in the final picture of world- 

 structure will transpire as the research proceeds; we do 

 not prejudge that. Actually we soon find that space- 

 lengths or time-lengths taken singly are relative, and only 



* As a recent illustration of this attitude I may refer to Bertrand 

 Russell's Analysis of Matter, p. 78 — a book with which I do not often 

 seriously disagree. "Whereas Eddington seems to regard it as necessary 

 to adopt Einstein's variable space, Whitehead regards it as necessary 

 to reject it. For my part, I do not see why we should agree with either 

 view; the matter seems to be one of convenience in the interpretation of 

 formulae." Russell's view is commended in a review by C. D. Broad. 

 See also footnote, p. 142. 



