398 Testimony. [CHAP. xvi. 



only data to which any one could appeal being that each 

 face turns up on an average once in six times. 



5. Let us now examine how far the above conditions 

 are fulfilled in the case of problems which discuss what is 

 called the credibility of testimony. The following would be 

 a fair specimen of one of the elementary enquiries out of 

 which these problems are composed; Here is a statement 

 made by a witness who lies once in ten times, what am I to 

 conclude about its truth? Objections might fairly be raised 

 against the possibility of thus assigning a man his place 

 upon a graduated scale of mendacity. This however we will 

 pass over, and will assume that the witness goes about the 

 world bearing stamped somehow on his face the appropriate 

 class to which he belongs, and consequently, the degree of 

 credit to which he has a claim on such general grounds. 

 But there are other and stronger reasons against the ad- 

 missibility of this class of problems. 



6. That which has been described in the previous 

 sections as the individual which had to be assigned to an 

 appropriate class or series of statistics is, of course, in this 

 case, a statement. In the particular instance in question this 

 individual statement is already assigned to a class, that 

 namely of statements made by a witness of a given degree of 

 veracity ; but it is clearly optional with us whether or not we 

 choose to confine our attention to this class in forming our 

 judgment; at least it would be optional whenever we were 

 practically called on to form an opinion. But in the case of 

 this statement, as in that of the mortality of the man whose 

 insurance we were discussing, there are a multitude of other 

 properties observable, besides the one which is supposed to 

 mark the given class. Just as in the latter there were 

 (besides his age), the place of his birth, the nature of his 

 occupation, and so on; so in the former there are (besides its 



