PHILOSOPHY AS AN ORGANON 



is an invariable antecedent of its crystallization." 

 In this particular case we may say that it is not 

 only the invariable antecedent, but the uncon- 

 ditional invariable antecedent, or cause ; since, 

 having detected the antecedent, we may pro- 

 duce it artificially, and find that the effect 

 follows it. It was thus in Sir James Hall's 

 splendid experiment, in which " he produced 

 artificial marble by the cooling of its materials 

 from fusion under immense pressure." And it 

 was thus when Dr. Wollaston, " by keeping a 

 vial of water charged with silicious particles 

 undisturbed for years, succeeded in obtaining 

 crystals of quartz." 



Manifestly, however, unless we can artifi- 

 cially produce the antecedent, and so reason 

 back from cause to effect, our method of agree- 

 ment is not exhaustively conclusive. Unless 

 we can be sure that the observed antecedent is 

 the only one common to all the instances, the 

 sequence may turn out to be only a derivative 

 sequence, like that of day and night. And unless 

 the phenomena are very simple, we cannot be 

 sure that the oberved common antecedent is the 

 only one. It is otherwise with the method of 

 difference. Whenever we can bring that method 

 to bear upon the phenomena, its results are finally 

 conclusive ; since it is the very essence of that 

 method to compare two instances which are ex- 

 actlv alike in everv respect save in the presence 

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