INTERMIXTURE OF EFFECTS. 321 



mercury, and known laws of the human body, and by reasoning from these, 

 would attempt to discover whether mercury will act upon the body when 

 in the morbid condition supposed, in such a manner as would tend to re- 

 store health. The experimental method would simply administer mercury 

 in as many cases as possible, noting the age, sex, temperament, and other 

 peculiarities of bodily constitution, the particular form or variety of the 

 disease, the particular stage of its progress, etc., remarking in which of 

 these cases it was attended with a salutary effect, and with what circum- 

 stances it was on those occasions combined. The method of simple obser- 

 vation would compare instances of recovery, to find whether they agreed 

 in having been preceded by the administration of mercury ; or would com- 

 pare instances of recovery with instances of failure, to find cases which, 

 agreeing in all other respects, differed only in the fact that mercury had 

 been administered, or that it had not. 



§ 7. That the last of these three modes of investigation is applicable to 

 the case, no one has ever seriously contended. No conclusions of value on 

 a subject of such intricacy ever were obtained in that way. The utmost 

 that could result would be a vague general impression for or against the 

 efficacy of mercury, of no avail for guidance unless confirmed by one of the 

 other two methods. Not that the results, which this method strives to ob- 

 tain, would not be of the utmost possible value if they could be obtained. 

 If all the cases of recovery Avhich presented themselves, in an examination 

 extending to a great number of instances, were cases in which mercury had 

 been administered, we might generalize with confidence from this expe- 

 rience, and should have obtained a conclusion of i-eal value. But no such 

 basis for generalization can we, in a case of this description, hope to obtain. 

 The reason is that which we have spoken of as constituting the character- 

 istic imperfection of the Method of Agreement, Plurality of Causes. Sup- 

 posing even that mercury does tend to cure the disease, so many other 

 causes, both natural and artificial, also tend to cure it, that there are sure 

 to be abundant instances of recovery in which mercury has not been ad- 

 ministered, unless, indeed, the practice be to administer it in all cases ; on 

 which supposition it will equally be found in the cases of failure. 



When an effect results from the union of many causes, the share which 

 each has in the determination of the effect can not in general be great, 

 and the effect is not likely, even in its presence or absence, still less in its 

 variations, to follow, even approximately, any one of the causes. Recov- 

 ery from a disease is an event to which, in every case, many influences 

 must concur. Mercury may be one such influence ; but from the very fact 

 that thej'e are many other such, it will necessarily happen that although 

 mercury is administered, the patient, for want of other concurring influ- 

 ences, will often not recover, and that he often will recover when it is 

 not administered, the other favorable influences being suflSciently powerful 

 without it. Neither, therefore, will the instances of recovery agree in the 

 administration of mercury, nor will the instances of failure agree in its 

 non- administration. It is much if, by multiplied and accurate returns 

 from hospitals and the like, we can collect that there are rather more re- 

 coveries and rather fewer failures when mercury is administered than when 

 it is not ; a result of very secondary value even as a guide to practice, and 

 almost worthless as a contribution to the theory of the subject.* 



* It is justly remarked by Professor Bain, that though the Methods of Agreement and Dif- 

 ference are not applicable to these cases, they are not wholly inaccessible to the Method of 



21 ; 



