320 INDUCTION. 



ception 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 astronomer knows, tend toward the earth, 

 with a force exactly equal to that with which the earth tends toward them. 

 The resistance of the atmosphere might, in the particular case of the bal- 

 loon, 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 tlie so-called exception, do not divide the cases 

 between them ; each of them is a comprehensive rule extending to all cases. 

 To call one of these concurrent principles an exception to the other, is su- 

 perficial, and contrary to the correct principles of nomenclature and ar- 

 rangement. 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 according to what method these complex 

 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 circum- 

 stances in which it may be expected again to occur. The conditions of a 

 phenomenon which arises from a composition of causes, may be investi- 

 gated either deductively or experimentally. 



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 

 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 call- 

 ed the method a priori. The other, or a posteriori method, professes to 

 proceed according to the canons of experimental inquiry. Considering 

 the whole assemblage of concurrent causes which produced the phenome- 

 non, as one single cause, it attempts to ascertain the cause in the ordinary 

 manner, by a comparison of instances. This second method subdivides 

 itself into two different varieties. If it merely collates instances 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 combination which will produce the given total effect, it is a method 

 of experiment. 



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 favorite maxim of Lord Chancellor Eldon, to 

 which, though it has often incurred philosophical ridicule, a deeper phi- 

 losophy will not refuse its sanction) to "clothe them in circumstances," 

 "We shall select for this purpose a case which as yet furnishes no very brill- 

 iant example of the success of any of the three methods, but which is all 

 the more suited to illustrate the difficulties inhei'ent in them. Let the sub- 

 ject 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 



..^t. 



J^Ksiays on some Unsettled Questions of Political Economy, Essay V. 



