460 INDUCTION. 



the experiment whether it could produce a); or as 

 an inferior resource, in case the Method of Difference 

 is impracticable ; which, as we before showed, gene- 

 rally arises from the impossibility of artificially pro- 

 ducing the phenomena. And hence it is that the 

 Method of Agreement, although applicable in principle 

 to either case, is more emphatically the method of 

 investigation on those subjects where artificial experi- 

 mentation is impossible ; because on those it is, gene- 

 rally, our only resource 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. Our next remark shall be, that there are 

 many cases in which, although our power of producing 

 the phenomenon is complete, the Method of Difference 

 either cannot 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 pheno- 

 menon depends, we can make no use, for that pur- 

 pose, of the Method of Difference ; for we cannot find 

 another substance precisely resembling Iceland spar 



