INTERMIXTURE OF EFFECTS. 



297 



by sufficient multiplication of experi- 

 ments, 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 in- 

 tend to try an experiment, we do not 

 reckon it enough that there be no cir- 

 cumstance 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 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 in- 

 fluence, 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 consider- 

 ing. The mercury of our experiment 

 being tried with an unknown multi- 

 tude (or even let it be a known mul- 

 titude) 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 know- 

 ing 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 sup- 

 pose the very problem solved which 

 we are considering the means of solv- 

 ing,) 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 pre- 

 ceded 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 tran- 



sition. As for the other mode of em- 

 ploying 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 phe- 

 nomena 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. 



Anything 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 favourable cases, only dis- 

 cover by a succession of trials that a 

 certain 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 exerts is not thwarted by other 

 tendencies in nearly as many cases as 

 it is fulfilled. Some causes indeed 

 there are which are more potent than 

 any counteracting causes to which 

 they are commonly exposed ; and ac- 

 cordingly 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, colchicum, lime-juice, cod- 

 liver oil," * and a few others. Even 

 these are not invariably followed by 

 success ; but they succeed in so large 

 a proportion of cases, and against 

 such powerful obstacles, that their 

 tendency to restore health in the dis- 

 orders for which they are prescribed 

 may be regarded as an experimental 

 truth.t 



* Bain's Logic, ii. 360. 



t What is said in the text on the inappli- 

 cability of the experimental methods tore- 

 solve particular questions of medical treat- 

 ment does not detract from their eflBcacv 

 in ascertaining the general laws of the ani- 

 mal or human system. The functions, for 

 example, of the different classes of nervea 



