308 CAUSATION 



amounts to disguising a piece of the unknown future as 

 an unknowable element of the present. Classical physics 

 foists a deterministic scheme on us by a trick; it smuggles 

 the unknown future into the present, trusting that we 

 shall not press an inquiry as to whether it has become 

 any more knowable that way. 



The same principle extends to every kind of pheno- 

 menon that we attempt to predict, so long as the need 

 for accuracy is not buried under a mass of averages. To 

 every co-ordinate there corresponds a momentum, and 

 by the principle of indeterminacy the more accurately 

 the co-ordinate is known the less accurately the momen- 

 tum is known. Nature thus provides that knowledge 

 of one-half of the world will ensure ignorance of the 

 other half — ignorance which, we have seen, may be 

 remedied later when the same part of the world is con- 

 templated retrospectively. We can scarcely rest content 

 with a picture of the world which includes so much that 

 cannot be known. We have been trying to get rid of 

 unknowable things, i.e. all conceptions which have no 

 causal connection with our experience. We have elimi- 

 nated velocity through aether, "right" frames of space, 

 etc., for this reason. This vast new unknowable element 

 must likewise be swept out of the Present. Its proper 

 place is in the Future because then it will no 

 longer be unknowable. It has been put in prematurely 

 as an anticipation of that which cannot be antici- 

 pated. 



In assessing whether the symbols which the physicist 

 has scattered through the external world are adequate to 

 predetermine the future, we must be on our guard 

 against retrospective symbols. It is easy to prophesy 

 after the event. 



