INTERMIXTURE OF EFFECTS. 503 



set of circumstances which we have exactly ascertained. It 

 needs hardly be remarked how far this condition is from being 

 realized in any case connected with the phenomena of life; 

 how far we are from knowing what are all the circumstances 

 which pre-exist in any instance in which mercury is admi- 

 nistered to a living being. This difficulty, however, though 

 insuperable in most cases, may not be so in all ; there are 

 sometimes concurrences of many causes, in which we yet know 

 accurately what the causes are. Moreover, the difficulty may 

 be attenuated by sufficient multiplication of experiments, in 

 circumstances rendering it improbable that any of the un- 

 known causes should exist in them all. But when we have got 

 clear of this obstacle, we encounter another still more serious. 

 In other cases, when we intend to try an experiment, we do 

 not reckon it enough that there be no circumstance in the 

 case the presence of which is unknown to us. We require 

 also that none of the circumstances which we do know, 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 subduction 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 multi- 

 tude) of other influencing 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 not. 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 con- 

 sidering 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 mercury. The Method 

 of Difference, in the ordinary mode of its use, namely by 



