216 Induction. [CHAP. IX. 



us and his death within the stated period. Regarded by 

 themselves there is nothing in common between them, and 

 therefore no link by which they may be connected or dis 

 connected with each other. The various classes above 

 referred to are a set of such middle terms, and the proposi 

 tions belonging to them are a corresponding set of major 

 premises. By the help of any one of them we are enabled, 

 under suitable circumstances, to connect together the subject 

 and predicate of the conclusion, that is, to infer whether the 

 man will or will not live twenty years. 



15. Now in the performance of such a logical process 

 there are two considerations to which the reader s attention 

 must for a moment be directed. They are simple enough in 

 this case, but will need careful explanation in the correspond 

 ing case in Probability. In the first place, it is clear that 

 whenever we can make any inference at all, we can do so 

 with absolute certainty. Logic, within its own domain, 

 knows nothing of hesitation or doubt. If the middle term 

 is appropriate it serves to connect the extremes in such a 

 way as to preclude all uncertainty about the conclusion ; 

 if it is not, there is so far an end of the matter : no conclu 

 sion can be drawn, and we are therefore left where we were. 

 Assuming our premises to be correct, we either know our 

 conclusion for certain, or we know nothing whatever about 

 it. In the second place, it should be noticed that none of 

 the possible alternatives in the shape of such major premises 

 as those given above can ever contradict any of the others, 

 or be at all inconsistent with them. Regarded as isolated 

 propositions, there is of course nothing to secure such har 

 mony ; they have very different predicates, and may seem 

 quite out of each other s reach for either support or opposi 

 tion. But by means of the other premise they are in each 

 case brought into relation with one another, and the general 



