THE DEDUCTIVE METHOD. 539 



Besides natural pathological facts, we can pro- 

 duce pathological facts artificially; we can try experi- 

 ments, even in the popular sense of the term, by 

 subjecting the living being to some external agent, 

 such as the mercury of our former example. As this 

 experimentation is not intended to obtain a direct 

 solution of any practical question, but to discover 

 general laws, from which afterwards the conditions of 

 any particular effect may be obtained by deduction ; 

 the best cases to select are those of which the circum- 

 stances 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 undisturbed, were it not for the disturbing 

 cause which we introduce. 



Such, with the occasional aid of the method of Con- 

 comitant Variations, (the latter not less encumbered 

 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 physiology ; in which indeed our knowledge 

 of causes is so imperfect, that we can neither explain, 

 nor could, without specific experience, have predicted 

 many of the facts which are certified to us by the 

 most ordinary observation. Fortunately, we are 



