SECT. 17.] Credibility of Extraordinary Stories. 423 



we thus anticipated it, would very seldom be justified, and 

 we are therefore apt to be much surprised when it does 

 occur. This, I take it, is the only legitimate sense of extra 

 ordinary so far as Probability is concerned. 



But there is another and very different use of the word, 

 which belongs to Induction, or rather to the science of 

 evidence in general, more than to that limited portion of it 

 termed Probability. In this sense the extraordinary, and 

 still more the improbable, event is not merely one of 

 extreme statistical rarity, which we could not expect to 

 guess aright, but which on moderate evidence we may pretty 

 readily accept; it is rather one which possesses, so to say, an 

 actual evidence-resisting power. It may be something which 

 affects the credibility of the witness at the fountain-head, 

 which makes, that is, his statements upon such a subject 

 essentially inferior to those on other subjects. This is the 

 case, for instance, with anything which excites his prejudices 

 or passions or superstitions. In these cases it would seem 

 unreasonable to attempt to estimate the credibility of the 

 witness by calculating (as in 6) how often his errors would 

 mislead us through his having been wrongly brought to an. 

 affirmation instead of adhering correctly to a negation. We 

 should rather be disposed to put our correction on the wit 

 ness average veracity at once. 



17. In true Probability, as has just been remarked, 

 every event has its own definitely recognizable degree of 

 frequency of occurrence. It may be excessively rare, rare to 

 any extreme we like to postulate, but still every one who 

 understands and admits the data upon which its occurrence 

 depends will be able to appreciate within what range of 

 experience it may be expected to present itself. We do not 

 expect it in any individual case, nor within any brief range, 

 but we do confidently expect it within an adequately exten- 



