428 Credibility of Extraordinary Stories. [CHAP. xvn. 



event in fact now become f$fi, which are enormously 

 greater than when there was only one witness. 



It appears therefore that when two, and of course still 

 more when many, witnesses agree in a statement in a matter 

 about which they might make many and various errors, the 

 combination of their favourable testimony adds enormously 

 to the likelihood of the event ; provided always that there 

 is no chance of collusion. And in the extreme case of the 

 opportunities for error being, as they well may be, practically 

 infinite in number, such combination would produce almost 

 perfect certainty. But then this condition, viz. absence of 

 collusion, very seldom can be secured. Practically our main 

 source of error and suspicion is in the possible existence of 

 some kind of collusion. Since we can seldom entirely get 

 rid of this danger, and when it exists it can never be sub 

 mitted to numerical calculation, it appears to me that combi 

 nation of testimony, in regard to detailed accounts, is yet 

 more unfitted for consideration in Probability than even that 

 of single testimony. 



21. The impossibility of any adequate or even appro 

 priate consideration of the credibility of miraculous stories 

 by the rules of Probability has been already noticed in 17. 

 But, since the grounds of this impossibility are often very 

 insufficiently appreciated, a few pages may conveniently be 

 added here with a view to enforcing this point. If it be 

 regarded as a digression, the importance of the subject and 

 the persistency with which various writers have at one time 

 or another attempted to treat it by the rules of our science 

 must be the excuse for entering upon it. 



A necessary preliminary will be to decide upon some defi 

 nition of a miracle. It will, we may suppose, be admitted by 

 most persons that in calling a miracle a suspension of a law 

 of causation, we are giving what, though it may not amount 



