INTERMIXTURE OF EFFECTS. 261 



characteristic imperfection of the Method of Agreement ; Plurality of 

 Causes. Supposing even that mercury does tend to cure the disease, 

 so many other causes, both natural and artificial, also teqd to cure it, 

 that there are sure to be abundant instances of recovery in which 

 mercury lias 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 iuilure. 



When an effect results from the union of many causes, the Bhare 

 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 onp. of the causes. 

 Recovery from a disease is an event to which, in every case, many inffu- 

 ences must concur. Mercury may be one such influence ; but, from 

 the Very fact t^iat there are many other such, it will- necessarily happen 

 that although mercury is administered, the patient, for want of other 

 concurring influences, will oft(>n not recover, and that he often will 

 recover when it is not administcu'ed, the other favorable influences 

 being sufficiently powerful v/ithout it. Neither, therefore, will the 

 instances of recovery agree in the adtainistration of mercury, nor will 

 the instances of failure agree in the non-administration of it. It is 

 much if, by multijilied 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 tlueory of the subject. 



§ 8. The inapplicability of the m.ethod of simple observation to 

 ascertain the conditions of effects dependent on many concurring causes, 

 being thus recognized ; we .shall next hiquire whether any greater 

 benefit can bo expected from the other branch of the a posteriori 

 method, that which proceeds by directly trying different combinations 

 of causes, either artificially produced or found in nature, and taking 

 notice what is their qffect : as, for example, by actually trying the effect 

 of ^mercury, in as many different circumstances as possible. This 

 method differs from the one which we have just examined, in turning 

 our attention directly to the causes or agents, instead of turning it to 

 the effect, recovery fi-om the disease. And since, as a general rule, 

 the effects of causes are far more accessible to our study than the causes 

 of effects, it is natural to think that this method may be successful 

 although the former must necessarily fail. 



The method now under consideration is called the Empirical Method ; 

 and in order to estimate it fairly, we must suppose it to be completely, 

 not incompletely, empirical. We must exclude from it (sverything 

 which partakes of the nature not of an experimental but of a deductive 

 operation. If for instance we try experiments with mercury upon a 

 person in health, in order to as(jcrtain the general laws of its- action 

 upon the human body, an<l then reason from these laws to determine 

 how it will act upon persons afft-ctod with a particular disease, this 

 may be a really effectual method, but this is Reduction. The experi- 

 mental method does not derive the law of a complex case from the 

 simpler laws which conspire to produce it, but makes its experiments 

 directly upon the complex case. Wo must make entire abstraction of 

 all knowledge of the simpler tendencies, the modi oj)erandi of mercury 



