PHYSICAL METHOD. 495 



depends on a concurrence of causes, these causes must be 

 studied one at a time, and their laws separately investigated, 

 if we wish, through the causes, to obtain the power of either 

 predicting or controlling the effect ; since the law of the effect 

 is compounded of the laws of all the causes which determine 

 it. The law of the centripetal and that of the projectile 

 force must have been known, before the motions of the earth 

 and planets could be explained, or many of them predicted. 

 The same is the case with the conduct of man in society. In 

 order to judge how he will act under the variety of desires and 

 aversions which are concurrently operating upon him, we must 

 know how he would act under the exclusive influence of each 

 one in particular. There is, perhaps, no action of a man's life 

 in which he is neither under the immediate nor under the 

 remote influence of any impulse but the mere desire of wealth. 

 With respect to those parts of human conduct of which wealth 

 is not even the principal object, to these political economy 

 does not pretend that its conclusions are applicable. But there 

 are also certain departments of human affairs, in which the 

 acquisition of wealth is the main and acknowledged end. It 

 is only of these that political economy takes notice. The 

 manner in which it necessarily proceeds is that of treating the 

 main and acknowledged end as if it were the sole end ; which, 

 of all hypotheses equally simple, is the nearest to the truth. 

 The political economist inquires, what are the actions which 

 would be produced by this desire, if within the departments in 

 question it were unimpeded by any other. In this way a 

 nearer approximation is obtained than would otherwise be 

 practicable to the real order of human affairs in those depart- 

 ments. This approximation has then to be corrected by 

 making proper allowance for the effects of any impulses of a 

 different description, which can be shown to interfere with the 

 result in any particular case. Only in a few of the most 

 striking cases (such as the important one of the principle of 

 population) are these corrections interpolated into the exposi- 

 tions of political economy itself; the strictness of purely scien- 

 tific arrangement being thereby somewhat departed from, for 

 the sake of practical utility. So far as it is known or may be 



