THE DISCOVERY OF LAWS 



instance must destroy the law. For an association which 

 has failed once, even if it has not failed a million times, 

 is not strictly invariable. True ; but what exactly is 

 the association we are asserting ? We arc asserting that 

 a certain density is invariably associated with a certain 

 substance. If we find a new density we cannot maintain 

 the invariable association if we attribute it to the same 

 substance as that to which the old density was attributed. 

 But why should we not attribute it to a new substance ? 

 If we try the experiment over agam and find that we do 

 not get the same result as before, what is to prevent us 

 avoiding any discrepancy between the two experiments 

 by simply saying that they are not made on the same 

 substance ? 



Indeed this way out of the difficulty has been adopted 



implicitly in the case of the black swan. Since we have 



known of black swans, we do not say that there are not 



white ^wans ; we recognize two kinds of swans, one of 



which is black and the other white. Nor do we recognise 



any error in the assertion, by those who did not know of 



black swans, that all swans are white. All the swans 



that they knew anything about were white and have 



remained white. The apparent difficulty arose 



only because the new birds were called swans. If we 



hat term to the birds which were originally called 



swans, any law about swans is quite unaffected by the 



discovery of birds which resemble swans in some i 



but which, since they are not wholly the B uld 



:>e called swanfc 



v be urged, the case is not really parallel to 



which we must suppose if we want to face 



diffi< ily. Black swans d;. a whit< in other 



thin. BO ih.'it thru- is a iva-on quite 



apart from their unexpected colour for distinguishing 



lie discovery 

 could -{'ill be found. 

 5 



