434 INDUCTION. 



refracting light, it was ascertained that they agree in the 

 circumstance of heing crystalline substances ; and though the 

 converse does not hold, though all crystalline substances have 

 not the property of double refraction, it was concluded, with 

 reason, that there is a real connexion between these two pro 

 perties ; that either crystalline 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 Agreement arises 

 a peculiar modification of that method, which is sometimes of 

 great avail in the investigation of nature. 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 agree 

 ing in every antecedent except A, or in every consequent except 

 a ; we may yet be able, by a double employment 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, bears testimony to a connexion between 

 A and a. In order to convert this evidence 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 

 A B C, to leave out A, and observe whether by doing so, a 

 is prevented. Now supposing (what is often the case) that we 

 are not able to try 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. Suppose, 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 esta 

 blished 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 



