322 INDUCTION. 



§ 8. The inapplicability of the method of simple observation to ascertain 

 the conditions of effects dependent on many concurring causes, being thus 

 recognized, we shall next inquire whether any greater benefit can be ex- 

 pected from the other branch of the a posteriori method, that which pro- 

 ceeds by directly trying different combinations of causes, either artificially 

 produced or found in nature, and taking notice what is their effect ; as, for 

 example, by actually trying the effect of mercury in as many different cir- 

 cumstances 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 from 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 has a 

 much better chance of proving successful than the former. 



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 every thing which par- 

 takes 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 ascertain the genei'al laws of its action upon the human body, 

 and then reason from these laws to determine how it will act uj3on persons 

 affected with a particular disease, this may be a really effectual method ; 

 but this is deduction. The experimental 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. We must make 

 entire abstraction of all knowledge of the simpler tendencies, the modi 

 operandi of mercury in detail. Our experimentation must aim at obtain- 

 ing a direct answer to the specific question. Does or does not mercury tend 

 to cure the particular disease ? 



Let us see, therefore, how far the case admits of the observance of those 

 rules of experimentation which it is found necessary to observe in other 

 cases. When we devise an experiment to ascertain the effect of a given 

 agent, there are certain precautions which we never, if we can help it, omit. 

 In the first place, we introduce the agent into the midst of a set of circum- 

 stances which Ave have exactly ascertained. It needs hardly be remarked 

 how far this condition is from being realized in any case connected with 

 the phenomena of life ; how far we are from knowing what are all the cir- 

 cumstances which pre-exist in any instance in which mercury is adminis- 

 tered to a living being. This difficulty, however, though insuperable in 



Concomitant Variations. " If a cause happens to vary alone, the effect will also vary alone : 

 a cause and effect may be thus singled out under the greatest complications. Thus, when the 

 appetite for food increases with the cold, we have a strong evidence of connection between 

 these two facts, although other circumstances may operate in the same direction. The as- 

 signing of the respective parts of the sun and moon in the action of the tides may be effected, 

 to a certain degree of exactness, by the variations of the amount according to the positions 

 of the two attractive bodies. By a series of experiments of Concomitant Variations, directed 

 to ascertain the elimination of nitrogen from the human body under varieties of muscular ex- 

 ercise. Dr. Parkes obtained the remarkable conclusion, that a muscle grows during exercise, 

 and loses bulk during the subsequent rest." {Logic, ii., 83.) 



It is, no doubt, often possible to single out the influencing causes from among a great num- 

 ber of mere concomitants, by noting what are the antecedents, a variation in which is followed 

 by a variation in the effect. But when there are many influencing causes, no one of them 

 greatly predominating over the rest, and especially when some of these are continually chan- 

 ging, it is scarcely ever possible to trace such a relation between the variations of the effect 

 and those of any one cause as would enable us to assign to that cause its real share in the 

 production of the effect. 



