THE FOUR EXPERIMENTAL METHODS. 451 



would seem, be the effect of A. Suppose, for exam- 

 ple, that A is tried along with B and C, and that the 

 effect is a b c; and suppose that A is next tried with 

 D and E, but without B and C, and that the effect is 

 ad e. Then we may reason thus : b and c are not 

 effects of A,, for they were not produced by it in the 

 second experiment ; nor are d and e y for they were 

 not produced in the first. Whatever is really the 

 effect of A must have been produced in both instances; 

 now this condition is fulfilled by no circumstance 

 except a. The phenomenon a cannot have been the 

 effect of B or C, since it was produced where they 

 were not; nor of D or E, since it was produced 

 where they were not. Therefore it is the effect of A. 

 For example, let the antecedent A be the contact 

 of an alkaline substance and an oil. This combination 

 being tried under several varieties of circumstance 

 resembling each other in nothing else, the results agree 

 in the production of a greasy and detersive or sapo- 

 naceous substance : it is therefore concluded that the 

 combination of an oil and an alkali causes the produc- 

 tion of a soap. It is thus we inquire, by the Method 

 of Agreement, into the effect of a given cause. 



In a similar manner we may inquire into the cause 

 of a given effect. Let a be the effect. Here, as shown 

 in the last chapter, we have only the resource of observa- 

 tion without experiment: we cannot take a phenomenon 

 of which we know not the origin, and try to find its 

 mode of production by producing it ; if we succeeded 

 in such a random trial it could only be by accident. 

 But if we can observe a in two different combinations, 

 abc and a d e; and if we know, or can discover, that 

 the antecedent circumstances in these cases respec- 

 tively were ABC and A D E ; we may conclude by a 

 reasoning similar to that in the preceding example, 



2 G 2 



