286 REALITY 



prestige of the world would be enhanced if it passed 

 the implied test. The external world is the world that 

 confronts that experience which we have in common, 

 and for us no other world could fill the same role, no 

 matter how high honours it might take in the qualifying 

 examination. 



This domestic definition of existence for scientific 

 purposes follows the principle now adopted for all other 

 definitions in science, namely, that a thing must be 

 defined according to the way in which it is in practice 

 recognised and not according to some ulterior signi- 

 ficance that we imagine it to possess. Just as matter 

 must shed its conception of substantiality, so existence 

 must shed its halo, before we can admit it into physical 

 science. But clearly if we are to assert or to question 

 the existence of anything not comprised in the external 

 world of physics, we must look beyond the physical 

 definition. The mere questioning of the reality of the 

 physical world implies some higher censorship than the 

 scientific method itself can supply. 



The external world of physics has been formulated 

 as an answer to a particular problem encountered in 

 human experience. Officially the scientist regards it as 

 a problem which he just happened across, as he might 

 take up a cross-word problem encountered in a news- 

 paper. His sole business is to see that the problem is 

 correctly solved. But questions may be raised about a 

 problem which play no part and need not be considered 

 in connection with the solving of the problem. The 

 extraneous question naturally raised about the problem 

 of the external world is whether there is some higher 

 justification for embarking on this world-solving com- 

 petition rather than on other problems which our 

 experience might suggest to us. Just what kind of 



