INTERMIXTURE OF EFFECTS. 323 



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. More- 

 over, the difficulty may be attenuated by sufficient multiplication of experi- 

 rjients, in circumstances rendering it improbable that any of the unknown 

 causes should exist in them all. But when we have got clear of this ob- 

 stacle, 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 re- 

 quire, also, that none of the circumstances which we do know shall have 

 effects susceptible of being confounded with those of the agents 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, af- 

 ter the subduction of those other effects, be apparent as a residual phe- 

 nomenon. 



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

 ering. The mercury of our experiment being tried with an unknown mul- 

 titude (or even let it be a known multitude) of other influencing circum- 

 stances, 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 considering the means of solving), we 

 can not 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 com- 

 paring the state of things following the experiment with the state which 

 preceded it, is thus, in the case of intermixture of effects, entirely unavail- 

 ing ; because other causes than that whose effect we are seeking to deter- 

 mine 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 chimerical. In phenomena so complicated it is question- 

 able if two cases, similar in all respects but one, ever occurred ; and were 

 they to occur, we could not possibly know that they were so exactly 

 similar. 



Any thing like a scientific use of the method of experiment, in these com- 

 plicated cases, is therefore out of the question. We can generally, even 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 these 

 conjunct effects, the portion which is determined by any one of the in- 

 fluencing agents, is usually, as we before remarked, but small ; and it must 

 be a more potent cause than most, if even the tendency which it really ex- 

 erts is not thwarted by other tendencies in nearly as many cases as it is ful- 

 filled. Some causes indeed there are. which are more potent than any 

 counteracting causes to which they are commonly exposed ; and according- 

 ly there are some truths in medicine which are sufficiently proved by direct 

 experiment. Of these the most familiar are those that relate to the efficacy 

 of the substances known as Specifics for particular diseases, " quinine, 

 colchicura, lime-juice, cod-liver oil,"* and a few others. Even these are 



* Bain's Logic, ii., 360. 



