426 INDUCTION. 



are d and e, 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 circumstances, resembling each other 

 in nothing else, the results agree in the production of a greasy 

 and detersive or saponaceous substance: it is therefore con 

 cluded that the combination of an oil and an alkali causes the 

 production 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 observation without 

 experiment : we cannot take a phenomenon of which we know 

 not the origin, and try to find its mode of production by pro 

 ducing it : if we succeeded in such a random trial it could only 

 be by accident. But if we can observe a in two different com 

 binations, a b c, and a d e ; and if we know, or can discover, 

 that the antecedent circumstances in these cases respectively 

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

 similar to that in the preceding example, that A is the ante 

 cedent connected with the consequent a by a law of causation. 

 B and C, we may say, cannot be causes of a, since on its 

 second occurrence they were not present ; nor are D and E, 

 for they were not present on its first occurrence. A, alone of 

 the five circumstances, was found among the antecedents of a 

 in both instances. 



For example, let the effect a be crystallization. We com 

 pare instances in which bodies are known to assume crystalline 

 structure, but which have no other point of agreement; and we 

 find them to have one, and as far as we can observe, only one, 

 antecedent in common : the deposition of a solid matter from 

 a liquid state, either a state of fusion or of solution. We con- 



