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CHAPTER VIII. 



OF THE FOUR METHODS OF EXPERIMENTAL 

 INQUIRY. 



1. THE simplest and most obvious modes of 

 singling out from among the circumstances which 

 precede or follow a phenomenon, those with which it 

 is really connected by an invariable law, are two in 

 number. One is, by comparing together different 

 instances in which the phenomenon occurs. The 

 other is by comparing instances in which the pheno- 

 menon does occur, with instances in other respects 

 similar in which it does not. These two methods 

 may be respectively denominated, the Method of 

 Agreement, and the Method of Difference. 



In illustrating these methods it will be necessary 

 to bear in mind the two-fold character of inquiries 

 into the laws of phenomena; which may be either 

 inquiries into the cause of a given effect, or into the 

 effects or properties of a given cause. We shall 

 consider the methods in their application to either 

 order of investigation, and shall draw our examples 

 equally from both. 



We shall denote antecedents by the large letters 

 of the alphabet, and the consequents corresponding to 

 them by the small. Let A, then, be an agent or 

 cause, and let the object of our inquiry be to ascertain 

 what are the effects of this cause. If we can either 

 find, or produce, the agent A in such varieties of 

 circumstances, that the different cases have no cir- 

 cumstance in common except A ; then, whatever 

 effect we find to be produced in all our trials must, it 



