PHILOSOPHY, THE GUIDE OF MENTAL LIFE 263 



observational experiments, which is reached when the observa- 

 tional or determining factors begin to interfere with the normal 

 happenings in the experiment under observation. 



The uncertainty principle having demonstrated the impossi- 

 bility of making critically accurate measurements at subatomic 

 levels, modern physicists, as mentioned above, transferred their 

 attention to "fields," which permitted them, in the words of Pro- 

 fessor Charles Galton Darwin, 2 "to carry through a formal mathe- 

 matical analogy without ever asking what it all meant in terms of 

 observable things . . . There grew up a great cult of doubting 

 the reality of unobserved things, and then a curious thing was 

 found; the charm did not work again . . . The work of the new 

 quantum theory has in fact run most surprisingly in the opposite 

 direction. The technique is largely concerned with wave func- 

 tions, which are much more abstract than anything in classical 

 mechanics. There is certainly nothing observable, or even pictur- 

 able, about waves propagating themselves in many-dimensional 

 space with absolutely unknowable phase, and with intensity con- 

 trolled by the curious extraneous rule of normalization. Largely 

 by use of these wave functions the whole of atomic physics has been 

 reduced to order, and so has molecular physics, except that it yields 

 problems in which so many electrons are interacting that a full dis- 

 cussion is not feasible.% So the doctrine of theorizing only about 

 observables was really not a useful doctrine; it merely provided a 

 geminating idea. In fact, we may well ask what an observable is, 

 and if we go at all beyond direct sensations, which as physicists 

 we certainly intend to do, the answer becomes perfectly indefinite. 

 This opinion I heard admirably expressed by the late Professor 

 Ehrenfest. It was in a physics meeting in Copenhagen and some 

 one was proposing a way out of certain difficulties which involved, 

 as he maintained, a reversion to the cult of the observable. Pro- 

 fessor Ehrenfest said: 'To believe that one can make physical 

 theories without metaphysics and without unobservable quanti- 

 ties, that is one of the diseases of childhood (das ist eine Kinder- 

 krankheit).' " 



Probability 



Chemical and physical kinetics recognize the impossibility of 

 predicting the behavior of single atoms or molecules, and base 

 their calculations on the laws of chance and probability, just as 

 do life insurance actuaries, who can never foretell just when anv 



