THE FOUR EXPERIMENTAL METHODS. 453 



subject to very considerable doubt. Though an inva- 

 riable, it may not be the unconditional antecedent of 

 a, but may precede it as day precedes night or night 

 day. This uncertainty arises from the impossibility 

 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 inva- 

 riable antecedents, we might be sure that the uncon- 

 ditional invariable antecedent, or cause, must be found 

 somewhere among them. Unfortunately it is hardly 

 ever possible to ascertain all the antecedents, unless 

 the phenomenon is one which we can produce artifi- 

 cially. 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 com- 

 pletely 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 pro- 

 duces 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 the phenomenon, or 

 can be absent notwithstanding its presence, is not 

 connected with it in the way of causation. The 

 casual circumstances being thus eliminated, if only 

 one remains, that one is the cause which we are in 

 search of : if more than one, they either are, or con- 



