METHOD OF DISCOVERING CAUSAL LAWS 179 



Again he means, of course, &quot; every circumstance in common save one,&quot; and 

 the circumstance of the successive presence and absence of the phenomenon 

 itself, which appears in one instance and not in the other. &quot;It is scarcely 

 necessary,&quot; he adds, &quot; to give examples of a logical process to which we 

 owe almost all the inductive conclusions we draw in early 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 fulness of life immediately before, all 

 circumstances being the same except the wound.&quot; He regards the method of 

 difference as the experimental method, par excellence, as the only one by which 

 &quot; we can ever, in the way of direct experience, arrive with certainty at causes.&quot; 1 

 He also gives the following example : &quot;If a bird is taken from a cage, and 

 instantly plunged into carbonic acid gas, the experimentalist may be fully as 

 sured (at all events after one or two repetitions) that no circumstance causing 

 suffocation had supervened in the interim, except the change from immersion 

 in the atmosphere to immersion in carbonic acid gas.&quot; 



Mill is inclined to overrate the value of this method. He failed to see that 

 of itself it does not enable us to analyse the phenomena under investigation 

 sufficiently to reach the one necessitating and indispensable cause. We require 

 something better than the method of single difference to carry us beyond that 

 less perfect stage of science in which we must admit plurality of causes. 



242. COMBINATION OF &quot; AGREEMENT &quot; AND &quot; DIFFERENCE &quot;. 

 As long as our methods of analysis merely enable us to assert 

 that something is a cause of a certain kind of phenomenon, we 

 have to recognize that the latter may have other causes ; and it 

 may be reasonably objected that the one we allege to be operative 

 in any given instance is not the one that is really operative there. 

 We endeavour to remove this uncertainty by combining the 

 methods of agreement and difference. This may be done either 

 when we are obliged to have recourse to simple observation alone, 

 or when we can make use of experiment. In the former case it 

 may be well to call this combined method the DOUBLE METHOD OF 

 AGREEMENT ; and in the latter to call it the JOINT METHOD 

 OF DIFFERENCE AND AGREEMENT. The former title will em 

 phasize the fact that although there is an element of the method 

 of difference present inasmuch as we have two sets of instances, 

 a positive set and a negative set yet we have not the essentials 

 of the method of difference, since we have to rely on simple obser 

 vation which does not give us any pair of positive and negative 

 instances differing in one respect only ; while the method of agree 

 ment predominates inasmuch as it is applied to each set of instances 

 successively. The latter title will emphasize the fact that the 

 method of difference is applied experimentally to the matter in 



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