530 INDUCTION. 



we do not reckon it enough that there be no circum- 

 stance in the case, the presence of which is unknown 

 to us. We require also that none of the circum- 

 stances which we do know of, shall have effects 

 susceptible of being confounded with those of the 

 agent whose properties we wish to study. We take 

 the utmost pains to exclude all causes capable of 

 composition with the given cause ; or if forced to let 

 in any such causes, we take care to make them such, 

 that we can compute and allow for their influence, so 

 that the effect of the given cause may, after the sub- 

 duction of those other effects, be apparent as a 

 residual phenomenon. 



These precautions are inapplicable to such cases 

 as we are now considering. The mercury of our 

 experiment being tried with an unknown multitude 

 {or even let it be a known multitude) of other influ- 

 encing circumstances, the mere fact of their being 

 influencing circumstances implies that they disguise 

 the effect of the mercury, and preclude us from 

 knowing whether it has any effect or no. Unless we 

 already knew what and how much is owing to every 

 other circumstance (that is, unless we suppose the 

 very problem solved which we are considering the 

 means of solving), we cannot tell that those other 

 circumstances may not have produced the whole of 

 the effect, independently or even in spite of the mer- 

 cury. The Method of Difference, in the ordinary 

 mode of its use, namely, by comparing the state of 

 things following the experiment with tbe state which 

 preceded it, is thus, in the case of intermixture of 

 effects, entirely unavailing ; because' other causes than 

 that whose effect we are seeking to determine, have 

 been operating during the transition. As for the 

 other mode of employing the Method of Difference, 



