262 INDUCTION. 



in de.tail ; our experimentation must aim at obtaining a direct answer 

 to the specific question, Does or does not mercuiy tend to cure the 

 particular disease 1 ' ■ 



Let us see, therefore, how far this case admits of the ohsen-ance of 

 those rules of experimentation, which it is found necessary to observe 

 "in other (iases. When we devise an experiment to ascertain the effect 

 of a given agent, there are certain precautions which we never, if we 

 can help it, omit. In the first place, we introduce the agent into the 

 midst of a 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 

 fi-om knowing what are all the citcumstances which preexist in any 

 instance in which mercury is administered to a living being. This 

 difficulty, however, though insuperable in most cases, may. not be so 

 in' all ; there are sometimes (though I should think never in physiology) 

 concurrences of many causes, in which we yet know accurately what 

 the causes are. But when w;e have got clear of this obstacle we en- 

 counter another still more serious. In other cases, when we intend to 

 try an experiment, we do not reckon it enovigh that there be no cir- 

 cumstance in the case, the presence of which is unknown to us. We 

 require also that none of the ciixumstances 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, thdt 

 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 appa- 

 rent as a residual phenomenon, 



These precautions are inapplicable to such cases as we are now con- 

 sidering. The mercury of our experiment being tried with an unknown 

 multitude (or even let it be a known multitude) 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 no. Unless we already knew 

 what and how much is owing to every other circumstance (that is, un- 

 less 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 mercury. The Method of Difference, in the ordinary mode 

 of its use, namely, by comparing the state of things following the ex- 

 periment with the state which preceded it, is thus, in the case of inter- 

 mixture 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, namely, by comparing, not the same case at two different 

 periods, but different cases, this in the present instance is quite chi- 

 merical. In phenomena so compliccfted it is questionable if two cases 

 similar in all respects but one ever occuiTed ; and were they to occur, 

 we could not possibly know that they were so exactly similar. 



Anything like a scientific use of the method of experiment, in these 

 complicated cases, is therefore out of the question. We can in the 

 most favorable cases only discover, by a succession of trials, that a cer- 

 tain cause is very often followed by a certain effect. For, in one of 



