258 



INDUCTION. 



this, if we can do it, is an application 

 of the Method of Difference, not of 

 the Method of Agreement. 



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 Agreement leads only to laws of 

 phenomena, (as some writers call them, 

 but improperly, since laws of causation 

 are also laws of phenomena,) that is, 

 to uniformities, which either are not 

 laws of causation, or in which the 

 question of causation must for the 

 present remain undecided. The 

 Method of Agreement is chiefly to 

 be resorted to as a means of suggest- 

 ing 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 antecedent 

 on which to try the experiment 

 whether it could produce a,) or as 

 an inferior resource in case the 

 Method of Difference is impracti- 

 cable ; which, as we before showed, 

 generally arises from the impossibility 

 of artificially producing the pheno- 

 mena. And hence it is that the 

 Method of Agreement, though appli- 

 cable in principle to either case, is 

 more emphatically the method o£ 

 investigation on those subjects where 

 artificial experimentation is impos- 

 sible ; because on those it is generally 

 our only resource of a directly induc- 

 tive nature ; while, in the phenomena 

 which we can produce at pleasure, 

 the Method of Difference generally 

 affords a more efl&cacious 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 com- 

 plete, 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 in- 

 quiry to be the cause of the double 

 refraction of light. "We can produce 

 this phenomenon at pleasure by em- 

 ploying any one of the many sub- 

 stances 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 phe- 

 nomenon depends, we can make no 

 use for that purpose of the Method 

 of Difference ; for we cannot find 

 another substance precisely resem- 

 bling Iceland spar except in some 

 one property. The only mode, there- 

 fore, of prosecuting this inquiry is 

 that afforded by the Method of Agree- 

 ment; by which, in fact, through a 

 comparison of all the known sub- 

 stances which have the property of 

 doubly refracting light, it was ascer- 

 tained that they agree in the circum- 

 stance of being 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 connection 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 pecu- 

 liar 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 agreeing in every 

 antecedent except A, or in every con- 

 sequent except a — we may yet be able, 

 by a double employment of the Me- 

 thod 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 



