222 INDUCTION. 



is only by setting in motion tlie exact process by which nature pro- 

 duces them ; and this being to us a mysterious process, of which the 

 main circumstances are not only unknown but unobservable, the name 

 of experimentation would here be completely misapplied. Such are 

 the facts : and what is the result 1 That on this vast subject, which 

 affords so much and such varied scope for observation, we have not, 

 properly -speaking, ascertained a single cause, a single unconditional 

 uliiformity. We know not, in the case of almost any of the phenom- 

 ena that we find conjoined, which is the condition of the other; which 

 is cause, and which effect, or whether either of them is so, or they are 

 not rather all of them" conjunct effects of causes yet to be discovered, 

 complex results of laws hitherto unknown. 



Although some of the foregoing observations may be, in technical 

 strictness of aiTangement, premature in this place, it seemed that a 

 few general remarks upon the difierence between Sciences of mere 

 Observation and Sciences of Experimentation, and the extreme disad- 

 vantage under which directly inductive inquiry is necessarily carried 

 on in the former, were the best preparation for discussing the methods 

 of direct induction ; a preparation rendering superfluous much that 

 must otherwise have been introduced, with some inconvenience, into 

 the heart of that discussion. To the consideration of these- Methods 

 we now proceed. 



CHAPTER VIII. 



OF THE FOUR METHODS OF EXPERIMENTAL INaUIRY. 



§ 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 

 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 

 eftects or properties of a given c^use. 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 con-esponding 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 produced in all our trials must, it would 

 seem, be the effect of A. Suppose, for example, that A is tried along 

 with B and C, and that the effect is abc; and suppose that A is next 



