306 CAUSATION 



ing through lack of information; it is a symbol for 

 causal failure — an indeterminacy of behaviour which is 

 part of the character of the atom. 



We have two chief ways of learning about the interior 

 of the atom. We can observe electrons entering or 

 leaving, and we can observe light entering or leaving. 

 Bohr has assumed a structure connected by strictly 

 causal law with the first phenomenon, Heisenberg and 

 his followers with the second. If the two structures were 

 identifiable then the atom w r ould involve a complete 

 causal connection of the two types of phenomena. But 

 apparently no such causal linkage exists. Therefore we 

 have to be content with a correlation in which the 

 entities of the one model represent probabilities in the 

 second model. There are perhaps details in the two 

 theories which do not quite square with this; but it 

 seems to express the ideal to be aimed at in describing 

 the laws of an incompletely causal world, viz. that the 

 causal source of one phenomenon shall represent the 

 probability of causal source of another phenomenon. 

 Schrodinger's theory has given at least a strong hint 

 that the actual world is controlled on this plan. 



The Principle of Indeterminacy. Thus far we have 

 shown that modern physics is drifting away from the 

 postulate that the future is predetermined, ignoring it 

 rather than deliberately rejecting it. With the discovery 

 of the Principle of Indeterminacy (p. 220) its attitude 

 has become more definitely hostile. 



Let us take the simplest case in which we think we 

 can predict the future. Suppose that we have a particle 

 with known position and velocity at the present instant. 

 Assuming that nothing interferes with it we can predict 

 the position at a subsequent instant. (Strictly the non- 



