^94 



INDUCTION. 



that the resistance of the atmosphere, 

 which prevents a balloon from falling, 

 constitutes the balloon an exception 

 to that pretended law of nature. But 

 the real law is, that all heavy bodies 

 tend to fall ; and to this there is 

 no exception, not even the sun and 

 moon ; for even they, as every astro- 

 nomer knows, tend towards the earth, 

 with a force exactly equal to that 

 with which the earth tends towards 

 them. The resistance of the atmos- 

 phere might, in the particular case of 

 the balloon, from a misapprehension 

 of what the law of gravitation is, be 

 said to prevail over the law ; but its 

 disturbing effect is quite as real in 

 every other case, since though it does 

 not prevent, it retards the fall of all 

 bodies whatever. The rule and the 

 so-called exception do not divide the 

 cases between them ; each of them as 

 a comprehensive rule extending to all 

 cases. To call one of these concurrent 

 principles an exception to the other, 

 is superficial, and contrary to the cor- 

 rect principles of nomenclature and 

 arrangement. An effect of precisely 

 the same kind, and arising from the 

 same cause, ought not to be placed in 

 two different categories, merely as 

 there does or does not exist another 

 cause preponderating over it." * 



§ 6. We have now to consider ac- 

 cording to what method these com- 

 plex effects, compounded of the effects 

 of many causes, are to be studied ; 

 how we are enabled to trace each 

 effect to the concurrence of causes in 

 which it originated, and ascertain the 

 conditions of its recurrence — the cir- 

 cumstances in which it may be ex- 

 pected again to occur. The conditions 

 of a phenomenon which arises from a 

 composition of causes may be investi- 

 gated either deductively or experi- 

 mentally. 



The case, it is evident, is naturally 

 susceptible of the deductive mode of 

 investigation. The law of an effect 

 of this description is a result of the 



* Essays on Some Unsettled Questions of 

 Political Economy, Essay V. 



laws of the separate causes on the 

 combination of which it depends, and 

 is therefore in itself capable of being 

 deduced from these laws. This is 

 called the method d priori. The 

 other, or d posteriori method, pro- 

 fesses to proceed according to the 

 canons of experimental inquiry. Con- 

 sidering the whole assemblage of con- 

 current causes which produced the 

 phenomenon as one single cause, it 

 attempts to ascertain the cause in 

 the ordinary manner, by a compari- 

 son of instances. This second method 

 subdivides itself into two different 

 varieties. If it merely collates in- 

 stances of the effect, it is a method 

 of pure observation. If it operates 

 upon the causes, and tries different 

 combinations of them, in hopes of 

 ultimately hitting the precise com- 

 bination which will produce the given 

 total effect, it is a method of experi- 

 ment. 



In order more completely to clear 

 up the nature of each of these three 

 methods, and determine which of 

 them deserves the preference, it will 

 be expedient (conformably to a fa- 

 vourite maxim of Lord Chancellor 

 Eldon, to which, though it has often 

 incurred philosophical ridicule, a 

 deeper philosophy will not refuse its 

 sanction) to " clothe them in circum- 

 stances." We shall select for this 

 purpose a case which as yet furnishes 

 no very brilliant example of the suc- 

 cess of any of the three methods, 

 but which is all the more suited to 

 illustrate the difficulties inherent in 

 them. Let the subject of inquiry be 

 the conditions of health and disease 

 in the human body, or (for greater 

 simplicity) the conditions of recovery 

 from a given disease ; and in order 

 to narrow the question still more, let 

 it be limited, in the first instance, to 

 this one inquiry, Is or is not some 

 particular medicament (mercury, for 

 instance) a remedy for the given 

 disease ? 



Now, the deductive method would 

 set out from known properties of mer- 

 cury and known laws of the human 



