THE DEDUCTIVE METHOD. 327 



that tendency operates alone, or in combination with no agencies but those 

 of which the effect can, from previous knowledge, be calculated and allow- 

 ed for. 



Accordingly, in the cases, unfortunately very numerous and important, 

 in which the causes do not suffer themselves to be separated and observed 

 apart, there is much difficulty in laying down with due certainty the induc- 

 tive foundation necessary to support the deductive method. This difficulty 

 is most of all conspicuous in the case of physiological phenomena ; it being 

 seldom possible to separate the different agencies which collectively com- 

 pose an organized body, without destroying the very phenomena which it 

 is our object to investigate: 



following life, in creatures we dissect, 



We lose it, in the moment we detect. 



And for this reason I am inclined to the opinion that physiology (greatly 

 and rapidly progressive as it now is) is embarrassed by greater natural dif- 

 ficulties, and is probably susceptible of a less degree of ultimate perfec- 

 tion, than even the social science ; inasmuch as it is possible to study the 

 laws and operations of one human mind apart from other minds, much less 

 imperfectly than we can study the laws of one organ or tissue of the hu- 

 man body apart from the other organs or tissues. 



It has been judiciously remarked that pathological facts, or, to speak in 

 common language, diseases in their different forms and degrees afford in 

 the case of physiological investigation the most valuable equivalent to ex- 

 perimentation properly so called ; inasmuch as they often exhibit to us a 

 definite disturbance in some one organ or organic function, the remaining 

 organs and functions being, in the first instance at least, unaffected. It is 

 true that from the perpetual actions and reactions which are going on 

 among all parts of the organic economy, there can be no prolonged disturb- 

 ance in any one function without ultimately involving many of the others ; 

 and when once it has done so, the experiment for the most part loses its 

 scientific value. All depends on observing the early stages of the derange- 

 ment; which, unfortunately, are of necessity the least marked. If, how- 

 ever, the organs and functions not disturbed in the first instance become 

 affected in a fixed order of succession, some light is thereby thrown upon 

 the action which one organ exercises over another: and we occasionally 

 obtain a series of effects which we can refer with some confidence to the 

 original local derangement ; but for this it is necessary that we should know 

 that the original derangement icas local. If it was what is termed consti- 

 tutional ; that is, if we do not know in what part of the animal economy it 

 took its rise, or the precise nature of the distui-bance which took place in 

 that part, we are unable to determine which of the various derangements 

 was cause and which effect ; which of them were produced by one another, 

 and which by the direct, though perhaps tardy, action of the original cause. 



Besides natural pathological facts, we can produce pathological facts ar- 

 tificially : we can ti*y experiments, 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, or the section of a nerve to ascertain the functions 

 of different parts of the nervous system. As this experimentation is not 

 intended to obtain a direct solution of any practical question, but to dis- 

 cover general laws, from which afterward the conditions of any particular 

 effect may be obtained by deduction, the best cases to select are those of 

 which the circumstances can be best ascertained : and such are generally 



