502 INDUCTION. 



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 arti- 

 ficially 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 suc- 

 cessful than the former. 



The method now under consideration is called the Empi- 

 rical Method ; and in order to estimate it fairly, we must sup- 

 pose it to be completely, not incompletely, empirical. We 

 must exclude from it everything 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 ascertain the general laws of its action upon 

 the human body, and then reason from these laws to determine 

 how it will act upon 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 com- 

 plex 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 ex- 

 perimentation must aim at obtaining 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. W T hen we devise an ex- 

 periment 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 



