SECT. 35.] Fallacies. 367 



out unfortunately, or to claim credit for it (in oneself) because 

 it has happened to succeed. Of course if the conclusion was 

 avowedly one of a probable kind we must be prepared with 

 complacency to accept a hostile event, or even a succession of 

 them ; it is not until the succession shows a disposition to 

 continue over long chat suspicion and doubt should arise, and 

 then only by a comparison of the degree of the assigned 

 probability, and the magnitude of the departure from it 

 which experience exhibits. For any single failure the reply 

 must be, the advice was sound (supposing, that is, that it 

 was to be justified in the long run), and I shall offer it again 

 under the same circumstances. 



35. The distinction drawn in the above instance 

 deserves careful consideration ; for owing to the wide differ 

 ence between the kind of propositions dealt with in Proba 

 bility and in ordinary Logic, and the consequent difference in 

 the nature of the proof offered, it is quite possible for argu 

 ments of the same general appearance to be valid in the 

 former and fallacious in the latter, and conversely. 



For instance, take the well-known fallacy which consists 

 in simply converting a universal affirmative, i.e. in passing 

 from All A is B to All B is A. When, as in common Logic, 

 the conclusion is to be as certain as the premise, there is not 

 a word to be said for such a step. But if we look at the 

 process with the more indulgent eye of Induction or Proba 

 bility we see that a very fair case may sometimes be made 

 out for it. The mere fact that Some B is A raises a cer 

 tain presumption that any particular B taken at random will 

 be an A. There is some reason, at any rate, for the belief, 

 though in the absence of statistics as to the relative fre 

 quency of A and B we are unable to assign a value to this 

 belief. I suspect that there may be many cases in which a 

 man has inferred that some particular B is an A on the 



