462 INDUCTION. 



this decisive experiment ; yet, provided we can by 

 any means discover what would be its result if we 

 could try it, the advantage will be the same. Sup- 

 pose, then, that as we previously examined a variety 

 of instances in which a occurred, and found them to 

 agree in containing A, so we now observe a variety 

 of instances in which a does not occur, and find 

 them agree in not containing A; which establishes, by 

 the Method of Agreement, the same connexion between 

 the absence of A and the absence of a, which was 

 before established between their presence. As, then, it 

 had been shown that whenever A is present a is present, 

 so it being now shown that when A is taken away a is 

 removed along with it, we have by the one proposition 

 A B C, a b c, by the other B C, b c, the positive and 

 negative instances which the Method of Difference re- 

 quires. Thus, if it be true that all animals which have a 

 well-developed respiratory system, and therefore aerate 

 the blood perfectly, agree in being warm-blooded, while 

 those whose respiratory system is imperfect do not 

 maintain a temperature much exceeding that of the 

 surrounding medium, we may argue from this two- 

 fold experience, that the change which takes place 

 in the blood by respiration is the cause of animal 

 heat. 



This method may be called the Indirect Method of 

 Difference, or the Joint Method of Agreement and 

 Difference ; and consists in a double employment of 

 the Method of Agreement, each proof being indepen- 

 dent of the other, and corroborating it. But it is not 

 equivalent to a proof by the direct Method of Differ- 

 ence. For the requisitions of the Method of Difference 

 are not satisfied, unless we can be quite sure either 

 that the instances affirmative of a agree in no antece- 

 dent whatever but A, or that the instances negative of 



