INTERMIXTURE OF EFFECTS. 501 



which 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 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 spoken of as constituting the characteristic imperfec- 

 tion 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 administered : unless, indeed, 

 the practice be to administer it in all cases ; on which suppo- 

 sition 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, even 

 approximately, any one of the causes. Recovery from a 

 disease is an event to which, in every case, many inluences 

 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 being sufficiently 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 recoveries 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. 



8. The inapplicability of the method of simple obser- 

 vation to ascertain the conditions of effects dependent on 



