198 MYSTICISM AND LOGIC 



A system may be called " practically isolated " during a 

 given period if, although there might be states of the rest 

 of the universe which would produce more than the 

 assigned margin of error, there is reason to believe that 

 such states do not in fact occur. 



Strictly speaking, we ought to specify the respect in 

 which the system is relatively isolated. For example, 

 the earth is relatively isolated as regards falling bodies, 

 but not as regards tides ; it is practically isolated as 

 regards economic phenomena, although, if Jevons' sun- 

 spot theory of commercial crises had been true, it would 

 not have been even practically isolated in this respect. 



It will be observed that we cannot prove in advance 

 that a system is isolated. This will be inferred from the 

 observed fact that approximate uniformities can be 

 stated for this system alone. If the complete laws for 

 the whole universe were known, the isolation of a system 

 could be deduced from them ; assuming, for example, 

 the law of universal gravitation, the practical isolation of 

 the solar system in this respect can be deduced by the 

 help of the fact that there is very little matter in its 

 neighbourhood. But it should be observed that isolated 

 systems are only important as providing a possibility of 

 discovering scientific laws ; they have no theoretical 

 importance in the finished structure of a science. 



The case where one event A is said to " cause " another 

 event B, which philosophers take as fundamental, is 

 really only the most simplified instance of a practically 

 isolated system. It may happen that, as a result of 

 general scientific laws, whenever A occurs throughout a 

 certain period, it is followed by B ; in that case, A and B 

 form a system which is practically isolated throughout 

 that period. It is, however, to be regarded as a piece of 

 good fortune if this occurs ; it will always be due to special 



