224 INDUCTION. 



elusion that it is the cause of a remains subject to very considerable 

 doubt. Though an invai-iable, it may not be the unconditional ante- 

 cedent of a, but may precede it as day precedes night or night day. 

 This uncertainty arises from the imjjossibility of assuring ourselves that 

 A is the only immediate antecedent common to both the instances. If 

 we could be certain of having ascertained all the invariable antece- 

 dents, we might be sure that the unconditional invariable antecedent, 

 or cause, mustbe found somewhere among them. Unfortunately it is 

 hardly ever possible to ascertain all the antecedents, unless the phe- 

 nomenon is one which we can produce artificially. Even then the 

 difficulty is merely lightened, not removed : men knew how to raise 

 water in pumps long before they adverted to what was really the 

 operating circumstance in the means they employed, namely, the 

 pressure of the atmosphere on the open surface of the water. It is, 

 however, much easier to analyze completely a set of arrangements 

 made by ourselves, than the whole complex mass of the agencies 

 which nature happens to be exerting at the moment when she produ- 

 ces any given phenomenon. We may overlook some of the material 

 circumstances in an experiment with an electrical machine ; but we 

 shall, at the worst, be better acquainted with them than with those of 

 a thunder-storm. 



The mode of discovering and proving laws of nature, which we 

 have now examined, proceeds upon the following axiom : Whatever 

 circumstance can be excluded, without prejudice to tl>e phenomenon, 

 or can be absent notwithstanding it presence, is not connected with it 

 in the way of causation. The casual circumstances being thus elimi- 

 nated, if only one remains, that one is the catise which we are in 

 search of: if more than one, they either are, or contain among them, 

 the cause : and so, mutatis mutandis, of the effect. As this method 

 proceeds by comparing different instances to ascertain in what they 

 agree, I have termed it the Method of Agreement : and we may 

 adopt as its regulating principle the following canon: — 



First Canon. 



If two or more instances of the phenomenon under investigation have 

 only one circumstance in common, the circumstance in which alone all 

 the instances agree, is the cause (or effect) of the given jxhenomenon. 



'• Quitting for the present the Method of Agreement, to which we 

 shall almost immediately return, we proceed to a still more potent 

 instrument of the investigation of nature, the Method of Difference, 



§ 2. In the Method of Agreement, we endeavored to obtain in- 

 stances which agreed in the given circumstance but differed in every 

 other : in the present method we require, on the contrary, two in- 

 stances resembling one another in every other respect, but differing in 

 the presence or absence of the phenomenon we wish to study. If our 

 object be to discover the effects of an agent A, we must procure A in 

 some set of ascertained circumstances, as ABC, and having noted the 

 effects produced, compare them with the efiect of the rernaining 

 circumstances B C, when A is absent. If the effect of A B C is ahc, 

 and the effect of BC, he, it is evident that the effect of A is a. So 



