4io ANTIENT METAPHYSICS. Book V. 



concerning Body, Magnitude, and Qiiantity Continuous and Dif- 

 crete*, his Syftem of Geometry would neverthelefs be a perfed com- 

 plete Syftem, and all his Demonftrations would ftand upon a firm and 

 folid bottom, and the Metaphyfician would be able to let him right 

 in Metaphyfics, though, perhaps, unable to follow him through his 

 Demonftrations. In like manner. Sir Ifaac Newton may be wrong 

 in his metaphyfical principles, upon which he has thought proper to 

 build his Aftronomy, and yet his fyftem be perfedly good ; and a 

 metaphyfician may be able to fhow that he has erred in thefe prin- 

 ciples, without being able to follow him through all the dedudions 

 that he has made concerning the Motions of the Celeftial Bodies, 

 not from thofe principles with which he had nothing to do, but frona 

 fads, and true aftronomical principles belonging to his Science. 



The great difcovery which Sir Ifaac Newton has made, and which 

 does him fo much honour, is the difcovery of the Law^ as it is 

 called, of the Planetary Motion. Till this difcoveiy was made, there 

 was no Science or Syftem of Aftronomy ; and, accordingly, before 

 Sir Ifaac, there was nothing more known concerning it, but a num- 

 ber of fads, without any connedion or dependence upon one ano- 

 ther. Among thefe are fome which are extremely curious, and ap- 

 pear very wonderful ; fuch as that very intricate and complicated 

 proportion, which Kepler difcovered betwixt the periods of the re- 

 volutions of the Planets, and their diftances from the centre ; for he 

 found out, by comparing the Motions of the feveral Planets with 

 one another, that the fquares of the periodical times were as the 

 cubes of their diftance from the Centre. But this he knew only 

 as a fad ; and it was referved to Sir Ifaac Newton to make a fcience 

 of this fad, by inveftigating and difcovering the Law of the Plane- 

 tary Motion, from which this wonderful property of it can, by ne- 

 ceflary confequence, be deduced : For there would be no Syftem 

 or Science of Aftronomy, if there were no eflential property of the 



Motion 



* See Cap. 3d and 4th, Lib. I. of this V^olume, 



