METHOD OF DISCOVERING CAUSAL LAWS 175 



infer with certitude that this circumstance is a necessitating cause, though not 

 that it is the only possible (or indispensable) cause, of the phenomenon. But 

 the &quot;if&quot; here points to a condition that is practically never fulfilled in the 

 unanalysed data presented in our sense experience as the raw material for 

 induction. 



The following is one of Mill s own illustrations of the method of agree 

 ment : &quot; For example, let the effect be crystallization. We compare in 

 stances in which bodies are known to assume crystalline structure, but which 

 have no other point of agreement ; and we find them to have one, and as far 

 as we can observe, only one, antecedent in common : the deposition of a 

 solid matter from a liquid state, either a state of fusion or of solution. We 

 conclude, therefore, that the solidification of a substance from a liquid state is 

 an invariable antecedent of its crystallization.&quot; 1 



The second method is called the METHOD OF DIFFERENCE, 

 or of single difference. It is stated as follows by Dr. 

 Mellone : 



&quot; WHEN THE ADDITION OF AN AGENT is FOLLOWED BY THE 



APPEARANCE, OR ITS SUBTRACTION BY THE DISAPPEARANCE, OF 

 A CERTAIN EVENT, OTHER CIRCUMSTANCES REMAINING THE 

 SAME, THAT AGENT IS CAUSALLY CONNECTED WITH THE EVENT.&quot; 2 

 This rule is an application of the principle that whatever 

 cannot be eliminated without interfering with the phenomenon is 

 causally connected with the latter. 3 We saw that the method of 

 agreement was a method mainly of observation, a method of dis 

 covering causal laws by suggesting these as hypotheses for verifica 

 tion. The present method is a method mainly of experiment, a 

 method of proving causal laws by the verification of hypotheses 

 already suggested. It compares a positive instance (in which the 

 phenomenon occurs) with a negative instance (in which the 

 phenomenon does not occur). It is only when we can produce 

 the positive and negative instances by experiment, as immediately 

 successive phases of the same general set of conditions and cir 

 cumstances, that the rule can be applied with any considerable 

 degree of success. And the reason is this : for the successful 

 application of the rule we must be sure that there is no other 

 difference between the two instances besides the presence of the sup 

 posed cause in the one and its absence in the other. But simple 

 observation can rarely, if ever, guarantee a certain conviction 



1 Logic, III., viii., i. 2 oj&amp;gt;. cit., p. 300. 



3 Mr. Joseph, emphasizing the fact that we are always led to the true cause in 

 directly, i.e. by eliminating what is not the cause, enunciates the present ground of 

 elimination in this way : &quot; Nothing is the cause of a phenomenon in the presence 

 of which it nevertheless fails to occur,&quot; taking &quot;cause,&quot; of course, as the strict, 

 reciprocating cause (op. cit,, p. 404). 



