88 DISSERTATION SECOND. [part i. 



effect. It was in this manner that he sought to determine 

 the laws of motion, and of the collision of bodies, in which 

 last all his conclusions were erroneous. From the same 

 source he deduced the existence of a plenum, and the con- 

 tinual preservation of the same quantity of motion in the 

 universe ; a proposition which, in a certain sense, is true, 

 but in the sense in which he understood it, is altogether 

 false. Reasonings of the kind which he employed may 

 possibly suit, as Bacon observed, with intelligences of a 

 higher order than man, but to his case they are quite inap- 

 plicable. Of the fruit of this tree nature has forbidden him 

 to eat, and has ordained, that, with the sweat of his brow, 

 and the labour of his hands, he should earn his knowledge 

 as well as his subsistence. 



Descartes, however, did not reject experiment altogether, 

 though he assigned it a very subordinate place in his phi- 

 losophy. By reasoning down from first principles, he tells 

 us that he was always able to discover the effects ; but the 

 number of different shapes which those effects might as- 

 sume was so great, that he could not determine, without 

 having recourse to experiment, which of them nature had 

 preferred to the rest. " We employ experiment," says 

 he, " not as a reason by which any thing is proved, for 

 we wish to deduce effects from their causes, and not con- 

 versely causes from their effects. We appeal to experi- 

 ence only, that out of innumerable effects which may be 

 produced from the same cause, we may direct our atten- 

 tion to one rather than another." It is wonderful, that 

 Descartes did not see what a severe censure he was here 

 passing on himself; of how little value the speculations 

 must be that led to conclusions so vague and indefinite ; 

 and how much more philosophy is disgraced by affording 

 an explanation of things which are not, than by not afford- 

 ing an explanation of things which are. 



