INTERMIXTURE OF EFFECTS. 527 



impression for or against the efficacy of mercury, of 

 no real avail for guidance unless confirmed by one of 

 the other two methods. Not that the results, which this 

 method strives to obtain, would not be of the utmost 

 possible value if they could be obtained. If all the cases 

 of recovery which presented themselves, in an examina- 

 tion extending to a great number of instances, were 

 cases in which mercury had been administered, we 

 might generalize with confidence from this experience, 

 and should have obtained a conclusion of real 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 so often spoken of as constituting 

 the characteristic imperfection of the Method of Agree- 

 ment; Plurality of Causes. Supposing 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 reco- 

 very in which mercury has not been administered : 

 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 cannot in general be great : and the effect 

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

 in its variations,, to follow very exactly any one of the 

 causes. Recovery 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 there are many other such, it will necessarily 

 happen that although mercury is administered, the 

 patient, for want of other concurring influences, will 

 often not recover, and that he often will recover when 

 it is not administered, the other favourable influences 



