296 



INDUCTION. 



§ 8, The inapplicability of the 

 method of simple observation to as- 

 certain the conditions of effects de- 

 pendent on many concurring causes 

 being thus recognised, we shall next 

 inquire whether any greater benefit 

 can be expected from the other branch 

 of the a posteriori method, that which 

 proceeds by directly trying different 

 combinations of causes, either artifici- 

 ally 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 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 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. 



may be thug 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 circum- 

 stances may operate in the same direction. 

 The assigning of the respectire parts of the 

 r.\\n 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 at- 

 tractive bodies. By a series of experi- 

 ments of Concomitant Variations, directed 

 to ascertain the elimination of nitrogen 

 from the human body under varieties of 

 muscular exercise, Dr. Parkes obtained 

 the remarkable conclusion that a muscle 

 grows during exercise, and loses bulk dur- 

 ing the subsequent rest."— Xojric, ii. 83. 



It is, no doubt, often possible to single 

 out the influencing causes from among a 

 great number of mere concomitants, by 

 noting what are the antecedents a varia- 

 tion in which is followed by a variation in 

 the effect. But when there are many in- 

 fluencing causes, no one of them greatly 

 predominating over the rest, and especi- 

 ally when some of these are continually 

 changing, it is scarcely ever possible to 

 trace such a relation between the varia- 

 tions of the eflfect and those of any one 

 cause as would enable us to assign to that 

 oause its real shar* in the production of 

 the effect. 



The method now under considera- 

 tion is called the Empirical Method ; 

 and in order to estimate it fairly, we 

 must suppose it to be completely, not 

 incompletely, empirical. We musfc 

 exclude from it everything which par- 

 takes of the nature not of an experi- 

 mental but of a deductive operation. 

 If, for instance, we try experiments 

 with mercury upon a person in health, 

 in order to ascertain the general laws 

 of its action upon the human body, 

 and then reason from these la-we to 

 determine how it will act upon per- 

 sons affected with a particular disease, 

 this may be a really effectual method, 

 but this is deduction. The eicperi- 

 mental method does not derive the 

 law of a complex case from the sim- 

 pler laws which conspire to produce 

 it, but makes its experiments directly 

 upon the complex case. We must 

 make entire abstraction of all know- 

 ledge of the simpler tendencies, the 

 modi operandi, of mercury in detail. 

 Our experimentation must aim at 

 obtaining a direct answer to the speci- 

 fic 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 experi- 

 ment 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 

 circumstances which we have exactly 

 ascertained. It needs hardly be re- 

 marked how far this condition is from 

 being realised in any case connected 

 with the phenomena of life ; how far 

 we are from knowing what are all the 

 circumstances which pre-exist in any 

 instance in which mercury is adminis- 

 tered to a living being. This diffi- 

 culty, however, though insuperable in 

 most cases, may not be so in all : 

 there are sometimes concurrences of 

 many causes, in which we yet know 

 accurately what the causes are. More- 

 over, the difficulty may be attenuated 



