146 MYSTICISM AND LOGIC 



tially incapable of being found. Therefore, it would seem, 

 the correlation with objects of sense, by which physics was 

 to be verified, is itself utterly and for ever un verifiable. 

 There are two ways of avoiding this result. 



(1) We may say that we know some principle a priori, 

 without the need of empirical verification, e.g. that our 

 sense-data have causes other than themselves, and that 

 something can be known about these causes by inference 

 from their effects. This way has been often adopted by 

 philosophers. It may be necessary to adopt this way to 

 some extent, but in so far as it is adopted physics ceases 

 to be empirical or based upon experiment and observa- 

 tion alone. Therefore this way is to be avoided as much 

 as possible. 



(2) We may succeed in actually defining the objects of 

 physics as functions of sense-data. Just in so far as 

 physics leads to expectations, this must be possible, since 

 we can only expect what can be experienced. And in so 

 far as the physical state of affairs is inferred from sense- 

 data, it must be capable of expression as a function of 

 sense-data. The problem of accomplishing this expres- 

 sion leads to much interesting logico-mathematical work. 



In physics as commonly set forth, sense-data appear 

 as functions of physical objects : when such-and-such 

 waves impinge upon the eye, we see such-and-such 

 colours, and so on. But the waves are in fact inferred 

 from the colours, not vice versa. Physics cannot be 

 regarded as validly based upon empirical data until the 

 waves have been expressed as functions of the colours 

 and other sense-data. 



Thus if physics is to be verifiable we are faced with the 

 following problem : Physics exhibits sense-data as func- 

 tions of physical objects, but verification is only possible 

 if physical objects can be exhibited as functions of sense- 



