THE FOUR EXPKRI.MENTAL METHODS. 225 



again, if we begiji at the otUcr end, and desire, to investigate the gause 

 of an .ctfect a, wo must select an instance, as abc, in which the effect 

 occurs, .and iiL which the antecedents were ABC, and we must look 

 out for another inatance in which the remaining circumstances, he, 

 occur without a. If the antecedents, in that instance, are BC, we 

 know that the cause of a must be A : either A alone, or A in conjunc- 

 tion with some of the other circumstances present. 



It is scarcely necessary to give examples of a logical process to 

 Avhich we owe almost all the inductive conclusions we draw in daily 

 life. When a man is shot through the heart, it is by this method we 

 know that it was the gun-shot which killed him : for he was in the 

 fullness of life immediately before, all' circumstances being the same, 

 except the wound. 



The axjoras which ai^e taken for gi-anted in this method are evidently 

 the following : Whatever antecedent camiot be excluded without pre- 

 venting the phenomenon, is the cause, or a condition, of that phenom- 

 enon ; Whatever consequent can be excluded, with no other differ- 

 ence in the antecedents than the absence of a particular one, is the 

 effect of that one. Instead of comparing diffei"ent instances of a jahe- 

 nomenon, to discover in what they agree, this method compares an 

 instance of its occurrence with an instance of its non-occuri'ence, to 

 discover in what they difler. The canon which is the regulating prin- 

 ciple of the Method of Difference may be expressed as follows : — 



Second Canon. 



If an instance in which the 'phenomenon under investigation occurs, 

 and an instance in which it does not occur, have every circumstance save 

 ane i?i common, that one occurring only in the former ; the circumstance 

 in which alone the two instances differ, is the effect, or cause, or a neces- 

 sary part oftflie caiise, of the phenomenon. 



§ 3, The two methods which we have now stated have many features 

 of resemblance, but there are also many distinctions between them. 

 Both are methods of elimination. This tcim (which is enrploycd in 

 the theory of equations to denote the process by which one after 

 another of the elements of a question is excluded, and the solution 

 made to depend upon the relation between the remaining elements 

 only,) is well suited to express the operation, analagous to this, which 

 has been understood since the time of Bacon to be the foundation of 

 experimental inquiry : namely, the successive exclusion of the various 

 circumstances wliich are found to accompany a phenomenon in a given 

 instance, in order to ascertain what are those among them' which can 

 be absent consistently with the existence of the phenomenon. The 

 Method of Agreement stands on the ground that whatever can be 

 eliminated, is not connected with the phenomenon by any law. The 

 Method of Difference has for its foundation, that whatever can not be 

 eliminated, is connected with the phenomenon by a law. 



Of these methods, that of Diffc;renco is more particularly a method 

 of artificial experiment ; while that of Agreement is more especially 

 the resource we employ where experimentation is impossible. A few 

 reflections will prove the fact, and point out the reason of it. 



It is inherent in the peculiar character of the Method of Difference, 

 Fp 



