THE FOUR EXPERIMENTAL METHODS. 



257 



tion to one another. In the spon- 

 taneous operations of nature there is 

 generally such complication and such 

 obscurity, they are mostly either on 

 so overwhelmingly large or on so in- 

 accessibly minute a scale, we are liO 

 ignorant of a great part of the facts 

 which really take place, and even 

 those of which we are not ignorant 

 are so multitudinous, and therefore so 

 seldom exactly alike in any two cases, 

 that a spontaneous experiment, of the 

 kind required by the Method of Dif- 

 ference, is commonly not to be found. 

 When, on the contrary, we obtain a 

 phenomenon by an artificial experi- 

 ment, a pair of instances such as the 

 method requires is obtained almost as 

 a matter of course, provided the pro- 

 cess does not last a long time. A 

 certain state of surrounding circum- 

 stances existed before we commenced 

 the experiment ; this is B C. We 

 then introduce A ; say, for instance, 

 by merely bringing an object from 

 another part of the room, before there 

 has been time for any change in the 

 other elements. It is, in short,J(as M. 

 Comte observes,) the very nature of 

 an experiment to introduce into the 

 pre-existing state of circumstances a 

 change perfectly definite. We choose 

 a previous state of things with which 

 we are well acquainted, so that no 

 unforeseen alteration in that state is 

 likely to pass unobserved ; and into 

 this we introduce, as rapidly as pos- 

 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 ab- 

 sence 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 repeti- 

 tions) that no circumstance capable of 

 causing suffocation had supervened 

 in the interim, except the change 

 from immersion in the atmosphere to 

 immersion in carbonic acid gas. There 

 js one doubt, indeed, which may re- 



main in some cases of this descrip- 

 tion ; the effect may have been pro- 

 duced 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 

 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 requisi- 

 tions of the Method of Difference ; 

 but that by the spontaneous opera- 

 tions 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 anything, 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 

 rtisult, 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 unascer- 

 tained. If A B C, A D E, A F G, 

 are all equally followed by a, then a 

 is an invariable consequent of A. If 

 abc,ad€, afg, all number A among 

 their antecedents, then A is connected 

 as an antecedent, by some invariable 

 law, with a. But to determine whe- 

 ther this invariable antecedent is a 

 cause, or this invariable conseqtient 

 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 anything, namely, 

 an instance in which the effect, a, has 

 come into existence, with no other 

 change in the pre-existing circum- 

 stances than the addition of A. And 



