THE FOUR EXPERIMENTAL METHODS. 227 



The reverse of this is the case with the Method of Agreement. 

 We do uot here require instances of so special and dcterininale a kind. 

 Any instances whatever, in which nature presents us with a phenom- 

 enon, may be examined for tlic purposes of this method ; and if all 

 such instances agree ia anything, a conchision of considerable value is 

 already jittained. We can sekhmi, indeed, be sure that this one point 

 of agreement is the only one ; but our ignorance does not, as irt the 

 Method of Difference, vitiate the conclusion ; the certainty of the 

 result, as far as it goes, is not affected. We have ascertained one 

 invariable antecedent or consequent, however many other invariable 

 antecedents or consequents may still remain unascertained. If ABC, 

 ADE, AFG, are all eqilally followed by a, then a is an invariable 

 f(':ascqaGntof A. If «ir, a.'tc, ({f^, all nunibcr A iimong their ante- 

 cedents, then A is connected as an antecedent, by some invariable 

 law, with a. But to detei-mine whether this invariable antecedent is 

 a cause, or this invariable consequent an effect, we must be able, in 

 addition, to produce the one by means of the other ; or, at least, to obtain 

 that which alone constitutes our assurance of having produced any- 

 thing, namely, an instance in which the effect, a, has come into exist- 

 ence, with no other change in the preexisting circumstances than the 

 addition of A. And this, if we can do it, is an application of the 

 Method of Difference, not of the Method of Agi-eement. 



It thus appears to be by the Method of Difference alone that we can 

 ever, in the way of direct experience, arrive with certainty at causes. 

 The Method of Agi-eement leads only to laws of phenomena, as Mr. 

 Whewell calls thenx, but which (since laws of causation are also laws 

 of phenomena) I prefer to designate as uniformities in which the ques- 

 tion of causation must for the present 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 com- 

 parison of ABC, ADE, AFQ, suggested that A was the antecedent 

 on which to try the experiment ^vhethe^ 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 Agi-eement, although applicable in principle to either case, is more 

 emphatically the method of investigation on those subjects where arti- 

 ficial expei-imentation is impossible ; because on those it is, generally, 

 our only resource of a directly inductive nature ; while, in the phenome- 

 na which we can produce at pleasure, the Method of Difference gene- 

 rally 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 Agi-eement. 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 pheijomenoai,,?it^ 

 pleasure, by employing any one of the many substances which ire' 



