240 WORLD BUILDING 



I feel stress in my muscles; one form of energy gives me 

 the sensation of warmth; the ratio of momentum to mass 

 is velocity, which generally enters into my experience 

 as change of position of objects. When I say that I feel 

 these things I must not forget that the feeling, in so far 

 as it is located in the physical world at all, is not in the 

 things themselves but in a certain corner of my brain. 

 In fact, the mind has also invented a craft of world- 

 building; its familiar world is built not from the dis- 

 tribution of relata and relations but by its own peculiar 

 interpretation of the code messages transmitted along 

 the nerves into its sanctum. 



Accordingly we must not lose sight of the fact that 

 the world which physics attempts to describe arises 

 from the convergence of two schemes of world-building. 

 If we look at it only from the physical side there is 

 inevitably an arbitrariness about the building. Given 

 the bricks — the 16 measures of world-structure — there 

 are all sorts of things we might build. Or we might 

 take up again some of the rejected lumber and build a 

 still wider variety of things. But we do not build 

 arbitrarily; we build to order. The things we build have 

 certain remarkable properties; they have these pro- 

 perties in virtue of the way they are built, but they also 

 have them because such properties were ordered. There 

 is a general description which covers at any rate most 

 of the building operations needed in the construction 

 of the physical world; in mathematical language the 

 operation consists in Hamiltonian differentiation of an 

 invariant function of the 16 measures of structure. I do 

 not think that there is anything in the basal relation- 

 structure that cries out for this special kind of com- 

 bination; the significance of this process is not in 

 inorganic nature. Its significance is that it corresponds 



