SECT. 15.] Credibility of Extraordinary Stories. 421 



15. That extraordinary stories are in many cases, pro 

 bably in a great majority of cases, less trustworthy than 

 others must be fully admitted. That is, if we were to make 

 two distinct classes of such stories respectively, we should 

 find that the same witness, or similar witnesses, were propor 

 tionally more often wrong when asserting the former than 

 when asserting the latter. But it does not by any means 

 appear to me that this must always be the case. We may 

 well conceive, for instance, that with some people the mere 

 fact of the story being of a very unusual character may make 

 them more careful in what they state, so as actually to add 

 to their veracity. If this were so we might be ready to 

 accept their extraordinary stories with even more readiness 

 than their ordinary ones. 



Such a supposition as that just made does not seem to me 

 by any means forced. Put such a case as this : let us sup 

 pose that two persons, one of them a man of merely ordinary 

 probity and intelligence, the other a scientific naturalist, 

 make a statement about some common event. We believe 



tion of the problem in 5 (to begin Here t and t measure respectively 



with the simplest case) is of course his trustworthiness in usual and 



as follows. Let p be the antecedent unusual events. As a formal solu- 



probability of the event, and t the tion this certainly meets the objec- 



measure of the truthfulness of the tions stated above in 14 and 15. 



witness; then the chance of his state- The determination however of t 



pt would demand, as I have remarked, 



ment being true is . , ; . . . 



pt+ (1 -p) (1 - t) continually renewed appeal to ex- 



This supposes him to lie as much perience. In any case the practical 



when the event does not happen as methods which would be adopted, if 



when it does. But we may meet any plans of the kind indicated above 



the cases supposed in the text by were resorted to, seem to me to differ 



assuming that t is the measure of very much from that adopted by the 



his veracity when the event does mathematicians, in their spirit and 



not happen, so that the above plan. 



formula becomes 



