OF NATURAL PHILOSOPHY. 155 



nomenon will decide whether it be or be not the 

 only cause : still more evidently, if it be present 

 contrariwise in the two cases, and the effect be 

 thereby reversed. But if its total presence or 

 absence only produces a change in the degree or 

 intensity of the phenomenon, we can then only 

 conclude that it acts as a concurrent cause or 

 condition with some other to be sought elsewhere. 

 In nature, it is comparatively rare to find instances 

 pointedly differing in one circumstance and agree- 

 ing in every other ; but when we call experiment to 

 our aid, it is easy to produce them ; and this is, in 

 fact, the grand application of experiments of enquiry 

 in physical researches. They become more valuable, 

 and their results clearer, in proportion as they pos- 

 sess this quality (of agreeing exactly in all their 

 circumstances but one), since the question put to 

 nature becomes thereby more pointed, and its an- 

 swer more decisive. 



(157.) 8th, If we cannot obtain a complete ne- 

 gative or opposition of the circumstance whose in- 

 fluence we would ascertain, we must endeavour to 

 find cases where it varies considerably in degree. 

 If this cannot be done, we may perhaps be able to 

 weaken or exalt its influence by the introduction of 

 some fresh circumstance, which, abstractedly con- 

 sidered, seems likely to produce this effect, and thus 

 obtain indirect evidence of its influence. But then 

 we are always to remember, that the evidence so 

 obtained is indirect, and that the new circumstance 

 introduced may have a direct influence of its own, 

 or may exercise a modifying one on some other 

 circumstance. 



