416 Credibility of Extraordinary Stories. [CHAP. xvn. 



not think that any one would feel a difficulty in thus ex 

 orbitantly discounting some particular assertion of a witness 

 whom in most other respects he fully trusted. 



10. There is one particular case which has been re 

 garded as a difficulty in the way of this treatment of the 

 problem, but which seems to me to be a decided confirma 

 tion of it ; always, be it understood, within the very narrow 

 and artificial limits to which we must suppose ourselves to 

 be confined. This is the case of a witness whose veracity is 

 just one-half; that is, one who, when a mere yes or no is 

 demanded of him, is as often wrong as right. In the case of 

 any other assigned degree of veracity it is extremely difficult 

 to get anything approaching to a confirmation from prac 

 tical judgment and experience. We are not accustomed to 

 estimate the merits of witnesses in this way, and hardly ap 

 preciate what is meant by his numerical degree of truthful 

 ness. But as regards the man whose veracity is one-half, we 

 are (as Mr C. J. Monro has very ingeniously suggested) only 

 too well acquainted with such witnesses, though under a 

 somewhat different name ; for this is really nothing else than 

 the case of a person confidently answering a question about 

 a subject-matter of which he knows nothing, and can there 

 fore only give a mere guess. 



Now in the case of the lottery with one prize, when the 

 witness whose veracity is one-half tells us that we have 

 gained the prize, we find on calculation that his testimony 

 goes for absolutely nothing ; the chances that we have got 

 the prize are just the same as they would be if he had never 

 opened his lips, viz. j^o- But clearly this is what ought 

 to be the result, for the witness who knows nothing about 

 the matter leaves it exactly as he found it. He is indeed, 

 in strictness, scarcely a witness at all ; for the natural func 

 tion of a witness is to examine the matter, and so to add 



