LOGIC AS THE ESSENCE OF PHILOSOPHY 35 



causation ? There are broadly three possible answers : 

 (1) that it is itself known a priori ; (2) that it is a 

 postulate ; (3) that it is an empirical generalisation from 

 past instances in which it has been found to hold. The 

 theory that causation is known a priori cannot be definitely 

 refuted, but it can be rendered very unplausible by 

 the mere process of formulating the law exactly, and 

 thereby showing that it is immensely more complicated 

 and less obvious than is generally supposed. The theory 

 that causation is a postulate, i.e. that it is something which 

 we choose to assert although we know that it is very 

 likely false, is also incapable of refutation ; but it is 

 plainly also incapable of justifying any use of the law 

 in inference. We are thus brought to the theory that 

 the law is an empirical generalisation, which is the view 

 held by Mill. 



But if so, how are empirical generalisations to be 

 justified ? The evidence in their favour cannot be 

 empirical, since we wish to argue from what has been 

 observed to what has not been observed, which can only 

 be done by means of some known relation of the observed 

 and the unobserved ; but the unobserved, by definition, 

 is not known empirically, and therefore its relation to 

 the observed, if known at all, must be known independ- 

 ently of empirical evidence. Let us see what Mill says 

 on this subject. 



According to Mill, the law of causation is proved by 

 an admittedly fallible process called " induction by simple 

 enumeration." This process, he says, " consists in 

 ascribing the nature of general truths to all propositions 

 which are true in every instance that we happen to know 

 of." 1 As regards its fallibility, he asserts that "the 

 precariousness of the method of simple enumeration is in 



1 Logic, book Hi., chapter iii., 2. 



