THE FOUR EXPERIMENTAL METHODS. 433 



remain undecided. The Method of Agreement is chiefly to 

 be resorted to, as a means of suggesting applications of the 

 Method of Difference (as in the last example the comparison 

 of A B C, A D E, A F G, suggested that A was the antece- 

 dent on which to try 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, generally arises 

 from the impossibility of artificially producing the phenomena. 

 And hence it is that the Method of Agreement, though appli- 

 cable in principle to either case, is more emphatically the 

 method of investigation on those subjects where artificial ex- 

 perimentation is impossible : because on those it is, generally, 

 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. There are, however, many cases in which, though 

 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 phenomenon depends, we can make no 

 use, for that purpose, of the Method of Difference ; 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 

 Agreement; by which, in fact, through a comparison of all 

 the known substances which have the property of doubly 

 VOL. i. 28 



