PRELUDE TO THE EPOCH OF NEWTON. 141 



deduced from assumed principles by a long chain 

 of reasoning, and not verified and confirmed at every 

 step by detailed and exact facts, has hardly a chance 

 of containing any truth. Descartes said that he 

 should think it little to show how the world is 

 constructed, if he could not also show that it must 

 of necessity have been so constructed. The more 

 modest philosophy which has survived the boast- 

 ings of his school is content to receive all its know- 

 ledge of facts from experience, and never dreams 

 of interposing its peremptory must be when nature 

 is ready to tell us what is. The a priori philoso- 

 pher has, however, always a strong feeling in his 

 favour among men. The deductive form of his 

 speculations gives them something of the charm 

 and the apparent certainty of pure mathematics; 

 and while he avoids that laborious recurrence to 

 experiments, and measures, and multiplied obser- 

 vations, which is irksome and distasteful to those 

 who are impatient to grow wise at once, every fact 

 of which the theory appears to give an explana- 

 tion, seems to be an unasked and almost an in- 

 fallible witness in its favour. 



My business with Descartes here is only with 

 his physical Theory of Vortices ; which, great as 

 was its glory at one time, is now utterly extin- 

 guished. It was propounded in his Principia Phi- 

 losophice, in 1644. In order to arrive at this 

 theory, he begins, as might be expected of him, from 

 reasonings sufficiently general. He lays it down as 



