THE FOUR EXPERIMENTAL METHODS. 283 



of a directly inductive nature; while, in the phenomena which we can 

 produce at pleasure, the Method of Difference generally affords a more 

 efficacious process, which will ascertain causes as well as mere laws. 



§ 4. There are, however, many cases in which, though our power of 

 producing the phenomenon is complete, the Method of Difference either 

 can not be made available at all, or not without a previous employment of 

 the Method of Agreement. This occurs when the agency by which we 

 can produce the phenomenon is not that of one single antecedent, but a 

 combination of antecedents, which we have no power of separating from 

 each other, and exhibiting apart. For instance, suppose the subject of 

 inquiry to be the cause of the double refraction of light. We can produce 

 this phenomenon at pleasure, by employing any one of the many substances 

 which are known to refract light in that peculiar manner. But if, taking 

 one of those substances, as Iceland spar, for example, we wish to determine 

 on which of the properties of Iceland spar this remarkable phenomenon 

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

 ence; for we can not find another substance precisely resembling Iceland 

 spar except in some one property. The only mode, therefore, of prosecu- 

 ting this inquiry is that afforded by the Method of Agreement ; by which, 

 in fact, through a comparison of all the known substances which have the 

 property of doubly refracting light, it was ascertained that they agree in 

 the circumstance of being crystalline substances ; and though the converse 

 does not hold, thuugh all crystalline substances have not the property of 

 double refraction, it was concluded, with reason, that there is a real con- 

 nection between these two properties ; 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 in- 

 vestigation 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 agreeing 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 

 connection between A and a. In order to convert this evidence of connec- 

 tion 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. N"ow supposing 

 (what is often the case) that we are not able to try this decisive experi- 

 ment; yet, provided we can by any means discover what would be its re- 

 sult if we could try it, the advantage will be the same. Suppose, then, 

 that as we previously examined a variety of instances in Avhich 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 connection 

 between the absence of A and the absence of a, which was before estab- 

 lished between their presence. As, then, it had been shown that whenever 

 A is present a is present, so, Jt being now shown that when A is taken 



