THE DEDUCTIVE METHOD. 511 



popular sense of the term, by subjecting the living being to 

 some external agent, such as the mercury of our former ex- 

 ample, or the section of a nerve to ascertain the functions of 

 different parts of the nervous system. As this experimenta- 

 tion is not intended to obtain a direct solution of any prac- 

 tical question, but to discover general laws, from which 

 afterwards the conditions of any particular effect may be ob- 

 tained by deduction ; the best cases to select are those of which 

 the circumstances can be best ascertained : and such are generally 

 not those in which there is any practical object in view. The 

 experiments are best tried, not in a state of disease, which is 

 essentially a changeable state, but in the condition of health, 

 comparatively a fixed state. In the one, unusual agencies are 

 at work, the results of which we have no means of predicting ; 

 in the other, the course of the accustomed physiological 

 phenomena would, it may generally be presumed, remain un- 

 disturbed, were it not for the disturbing cause which we 

 introduce. 



Such, with the occasional aid of the Method of Concomi- 

 tant Variations, (the latter not less incumbered than the more 

 elementary methods by the peculiar difficulties of the subject,) 

 are our inductive resources for ascertaining the laws of the 

 causes considered separately, when we have it not in our power 

 to make trial of them in a state of actual separation. The 

 insufficiency of these resources is so glaring, that no one can 

 be surprised at the backward state of the science of physio- 

 logy ; in which indeed our knowledge of causes is so imperfect, 

 that we can neither explain, nor could without specific expe- 

 rience have predicted, many of the facts which are certified to 

 iis by the most ordinary observation. Fortunately, we are 

 much better informed as to the empirical laws of the pheno- 

 mena, that is, the uniformities respecting which we cannot 

 yet decide whether they are cases of causation, or mere results 

 of it. Not only has the order in which the facts of organiza- 

 tion and life successively manifest themselves, from the first 

 germ of existence to death, been found to be uniform, and 

 very accurately ascertainable ; but, by a great application of 

 the Method of Concomitant Variations to the entire facts of 



