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 phenomenon 

 does occur, with instances in other respects similar in which 

 it does not. These two methods may be respectively deno- 

 minated, the Method of Agreement, and the Method of Dif- 

 ference. 



In illustrating these methods, it will be necessary to bear 

 in mind the twofold 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 circumstance 

 in common except A ; then whatever effect we find to be pro- 

 duced in all our trials, is indicated as the effect of A. Sup- 

 pose, for example, 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 a d 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 



