43 INDUCTION. 



being conclusively tested by other experiments. It thus 

 appears that in the study of the various kinds of phenomena 

 which we can, by our voluntary agency, modify or control, we 

 can in general satisfy the requisitions of the Method of Dif- 

 ference ; but that by the spontaneous operations of nature 

 those requisitions are seldom fulfilled. 



The reverse of this is the case with the Method of Agree- 

 ment. We do not here require instances of so special and deter- 

 minate a kind. Any instances whatever, in which nature 

 presents us with a phenomenon, may be examined for the 

 purposes of this method ; and if all such instances agree in 

 anything, a conclusion of considerable value is already attained. 

 We can seldom, indeed, be sure that the one point of agree- 

 ment is the only one ; but this ignorance does not, as in 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 A B 0, A D E, A F G, are all equally 

 followed by a, then a is an invariable consequent of A. If 

 a b c, a d e, afg, all number A among their antecedents, then 

 A is connected as an antecedent, by some invariable law, 

 with a. But to determine 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 assur- 

 ance of having produced anything, namely, an instance in 

 which the effect, a, has come into existence, with no other 

 change in the pre-existing 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 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 im- 

 properly, 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 



