SECT. 5.] Credibility of Extraordinary Stories. 409 



by considerations of this nature, we should act very foolishly 

 if we did not adopt some such plan. Two methods of thus 

 correcting the average may be suggested : one of them being 

 that which practical sagacity would be most likely to employ, 

 the other that which is almost universally adopted by writers 

 on Probability. The former attempts to make the correction 

 by the following considerations : instead of relying upon the 

 witness general average, we assign to it a sort of conjectural 

 correction to meet the case before us, founded on our expe 

 rience or observation ; that is, we appeal to experience to 

 establish that stories of such and such a kind are more or 

 less likely to be true, as the case may be, than stories in 

 general. The other proceeds upon a different and some 

 what more methodical plan. It is here endeavoured to 

 show, by an analysis of the nature and number of the 

 sources of error in the cases in question, that such and such 

 kinds of stories must be more or less likely to be correctly 

 reported, and this in certain numerical proportions. 



5. Before proceeding to a discussion of these methods 

 a distinction must be pointed out to which writers upon the 

 subject have not always attended, or at any rate to which 

 they have not generally sufficiently directed their readers 

 attention 1 . There are, broadly speaking, two different ways 

 in which we may suppose testimony to be given. It may, in 

 the first place, take the form of a reply to an alternative 

 question, a question, that is, framed to be answered by yes 

 or no. Here, of course, the possible answers are mutually 

 contradictory, so that if one of them is not correct the other 

 must be so : Has A happened, yes or no ? The common 

 mode of illustrating this kind of testimony numerically is by 



1 I must plead guilty to this make the treatment of this part of 

 charge myself, in the first edition the subject obscure and imperfect, 

 of this work. The result was to and in some respects erroneous. 



