228 INDUCTION. 



known to refract light in tliat peculiar manner. But if, taking one of 

 those substances, as Iceland spar for example, we wish to detcmiine 

 on which of the properties of Iceland spar this remarkable phenomena 

 depends, we can make no use, for that purpose; of the Method of Dif- 

 ference ; for we cannot find another substance precisely resembling 

 Iceland spar except in some one property. The only mode, therefore, 

 of prosecuting this inquiry is that afforded by the Method of Agree- 

 ment ; by which, in fact, through a comparison of all the known sub- 

 stances Avhich had the property of doubly refracting light, it was ascer- 

 tained that they agreed in the single circumstance of being crystaline 

 substances ; and althoiigh the converse does not hold, although all crys- 

 taline substances have not the property of double refraction, it was 

 concluded, with reason, that there is a real connexion between these 

 two properties ; that either crystaline structure, or the cause which 

 gives rise to that structure, is one of the conditions of double refraction. 



Out of this employment of the Method of Agi'eement arises a pecu- 

 liar modification of that method,.- which is sometimes of great avail in 

 the investigation of jiature. In cases similar to the above, in which it 

 is not possible" to obtain the precise pair of instances which our second 

 canon requires — instances agreeing in every antecedent except A, or in 

 every consequent except a ; we may yet be able, by a. double employ- 

 ment of the Method of Agreement, to discover in what the instances 

 which contain A or a, differ from those which do not. 



If we compare various instances in which a occurs, and find that 

 they all have in common the circumstance A, and (as far as can be 

 observed) no other circumstance, the Method of Agreement, so far beais 

 testimony to a connexion between A and a. In order to convert this 

 proof of connexion into proof of causation by the direct' Method of 

 Difference, we ought to be able in some one of these instances, as for 

 example ABC, to leave out A, and observe whether by doing so, « is 

 prevented. Now supposing (what is often the case) that. we are not 

 able to try this decis\\'e experiment; yet, provided we can by any 

 means discover what would be its result if we could try it, the advan- 

 tage will be the same. Suppose, then, that as we previously examined 

 a variety of instances in which a occuired, 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 innoi containing A; which establishes, 

 by the Method of Agi-eeraent, the same connexion between the absence 

 of A and the absence qf'aj 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 re- 

 moved along with it, we bave by the one proposition ABC, abc, by 

 .the other BC, ic, the positive and negative instances which the Method 

 of Difference requires. 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 fi'om this two-fold expe- 

 rience, 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- 



