ON THE NOTION OF CAUSE 197 



course, less probability than the assumption that this or 

 that particular law is permanent ; and the assumption 

 that a particular law is permanent for all time has less 

 probability than the assumption that it will be valid up 

 to such and such a date. Science, in any given case, will 

 assume what the case requires, but no more. In con- 

 structing the Nautical Almanac for 1915 it will assume 

 that the law of gravitation will remain true up to the end 

 of that year ; but it will make no assumption as to 1916 

 until it comes to the next volume of the almanac. This 

 procedure is, of course, dictated by the fact that the 

 uniformity of nature is not known a priori, but is an 

 empirical generalisation, like "all men are mortal." In 

 all such cases, it is better to argue immediately from the 

 given particular instances to the new instance, than to 

 argue by way of a major premiss ; the conclusion is only 

 probable in either case, but acquires a higher probability 

 by the former method than by the latter. 



In all science we have to distinguish two sorts of laws : 

 first, those that are empirically verifiable but probably 

 only approximate ; secondly, those that are not verifiable, 

 but may be exact. The law of gravitation, for example, 

 in its applications to the solar system, is only empirically 

 verifiable when it is assumed that matter outside the 

 solar system may be ignored for such purposes ; we 

 believe this to be only approximately true, but we cannot 

 empirically verify the law of universal gravitation which 

 we believe to be exact. This point is very important in 

 connection with what we may call " relatively isolated 

 systems." These may be defined as follows : 



A system relatively isolated during a given period is 

 one which, within some assignable margin of error, will 

 behave in the same way throughout that period, however 

 the rest of the universe may be constituted. 



