282 INDUCTION. 



sible, the phenomenon which we wish to study ; so that in general we are 

 entitled to feel complete assurance that the pre-existing state, and the state 

 which we have produced, differ in nothing except the presence or absence 

 of that phenomenon. If a bird is taken from a cage, and instantly plunged 

 into carbonic acid gas, the experimentalist may be fully assured (at all 

 events after one or two repetitions) that no circumstance capable of caus- 

 ing suffocation had supervened in the interim, except the change from im- 

 mersion in the atmosphere to immersion in carbonic acid gas. There is 

 one, doubt, indeed, which may remain in some cases of this description ; 

 the effect may have been produced not by the change, but by the means 

 employed to produce the change. The possibility, however, of this last 

 supposition generally admits of being conclusively tested by other experi- 

 ments. It thus appears that in the study of the various kinds of phenome- 

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

 general satisfy the requisitions of the Method of Difference ; but that by 

 the spontaneous operations of nature those requisitions are seldom fulfilled. 



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

 not here require instances of so special and determinate a kind. Any in- 

 stances 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 any thing, a conclusion of considerable value is already attained. We 

 can seldom, indeed, be sure that the one point of agreement 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 unas- 

 certained. If A B C, A D E, A F G, are all equally followed by «, then a is an 

 invariable consequent of A. If ahc, ade, 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, 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 improperly, since laws of causation are also laws of phenom- 

 ena) : 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 sug- 

 gesting applications of the Method of Difference (as in the last example 

 the comparison of ABC, A D E, A P^ G, suggested that A was the ante- 

 cedent 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 ar- 

 tificially producing the phenomena. And hence it is that the Method of 

 Agreement, though applicable in principle to either case, is more emphat- 

 ically the method of investigation on those subjects where artificial experi- 

 mentation is impossible ; because on those it is, generally, our only resource 



