THE FOUR EXPERIMENTAL METHODS. 459 



attained. We can seldom, indeed, be sure that this 

 one point of agreement is the only one ; but our igno- 

 rance 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 ABC, ADE, AFG, are 

 all equally followed by a, then a is an invariable con- 

 sequent 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 assurance of having produced any- 

 thing, namely, an instance in which the effect, cf, 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 Agree- 

 ment. 



It thus appears to be by the Method of Difference 

 alone that we can ever, in the way of direct expe- 

 rience, arrive with certainty at causes. The Method of 

 Agreement leads only to laws of phenomena, as Mr. 

 Whewell calls them, but which (since laws of causation 

 are also laws of phenomena) I prefer to designate as 

 uniformities in which the question of causation must for 

 the present remain undecided. The Method of Agree- 

 ment 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, ADE, AFG, 

 suggested that A was the antecedent on which to try 



