THE FOUR EXPERIMENTAL METHODS. 



255 



subject to very considerable doubt. 

 Though an invariable, it may not be 

 the unconditional antecedent of a, 

 but may precede it as day precedes 

 night or night day. This uncertainty 

 arises from the impossibility of assur- 

 ing ourselves that A is the only imme- 

 diate antecedent common to both the 

 instances. If we could be certain of 

 having ascertained all the invariable 

 antecedents, we might be sure that 

 the unconditional invariable ante- 

 cedent or cause must be found some- 

 where among them. Unfortunately 

 it is hardly ever possible to ascertain 

 all the antecedents, unless the pheno- 

 menon is one which we can produce 

 artificially. Even then, the difficulty 

 is merely lightened, not removed : 

 men knew how to raise water in 

 pumps long before they adverted to 

 what was really the operating cir- 

 cumstance in the means they em- 

 ployed, namely, the pressure of the 

 atmosphere on the open surface of 

 the water. It is, however, much 

 easier to analyse completely a set 

 of arrangements matle by ourselves, 

 than the whole complex mass of the 

 agencies which nature happens to be 

 exerting at the moment of the pro- 

 duction of a given phenomenon. We 

 may overlook some of the material 

 circumstances in an experiment with 

 an electrical machine ; but we shall, 

 at the worst, be better acquainted 

 with them than with those of a 

 thunderstorm. 



The mode of discovering and prov- 

 ing laws of nature, which we have 

 now examined, proceeds on the follow- 

 ing axiom. Whatever circumstances 

 can be excluded, without prejudice to 

 the phenomenon, or can be absent 

 notwithstanding its presence, is not 

 connected with it in the way of causa- 

 tion. The casual circumstance being 

 thus eliminated, if only one remains, 

 that one is the cause which we are 

 in search of : if more than oiie, they 

 either are, or contain among them, 

 the cause ; and so, mutatis mutandis, 

 of the effect. As this method pro- 

 ceeds by comparing different instances 



to ascertain in what they agree, I have 

 termed it the Method of Agreement ; 

 and we may adopt as its regulating 

 principle the following canon : — 



First Canon. 



If two or more instances of the phe- 

 nomenon under investigation have only 

 one circumstance in common,the circum- 

 stance in which alone all the instances 

 agree is the cause {or effect) of the given 

 phenomenon. 



Quitting for the present the Method 

 of Agreement, to which we shall 

 almost immediately return, we pro-^ 

 ceed to a still more potent instrument 

 of the investigation of nature, the 

 Method of Difference. 



§ 2. In the Method of Agreement, 

 we endeavoured to obtain instances 

 which agreed in the given circum- 

 stance but differed in every other : in 

 the present method we require, on the 

 contrary, two instances resembling 

 one another in every other respect, 

 but differing in the presence or 

 absence of the phenomenon we wish 

 to study. If our object be to discover 

 the effects of an agent A, we must 

 prcxsure A in some set of ascertained 

 circumstances, as A B C, and having 

 noted the effects produced, compare 

 them with the effect of the remaining 

 circumstances B C, when A is absent 

 If the effect of A B C is a 6 c, and the 

 effect of B C, 6 c, it is evident that the 

 effect of A is a. So again, if we be- 

 gin at the other end, and desire to 

 investigate the cause of an effect a, 

 we must select an instance, as a 6 c, in 

 which the effect occurs, and in which 

 the antecedents were ABC, and we 

 must look out for another instance in 

 which the remaining circumstances, 

 h c, occur without a. If the antece- 

 dents, in that instance, are B C, we 

 know that the cause of a must be A : 

 either A alone, or A in conjunction 

 with some of the other circumstances 

 present. 



It is scarcely necessary to give ex- 

 amples of a logical process to which 

 we owe almost all the inductive con- 



