438 ANTIENT METAPHYSICS. BookV. 



wherever there is Body or Matter, there muft, according to thefc 

 philofophers, be Gravitation. And Mr. Cotes has gone fo far as to 

 maintain, in his Preface to the Principia, that Gravitation is effential 

 to Matter, though Sir Ifaac has faid, in fo many words, that he did 

 not affirm it to be fo : And Dr. PrieRley has maintained, not only 

 that Attradlion is eflential to Matter, but that it and Repulfion con- 

 ftitute its nature and effence, not extenfion, foUdity, or impenc- 

 trabiUty, as has been commonly underftood. All thefe errors have 

 arifen from two fources ; JirJJ, fuppofmg that the hypothefes of Sir 

 Ifaac were realities, not mere hypothefes for the fake of teaching 

 and demonftration ; and, fecoiidl)\ that Sir Ifaac intended, not only 

 to difcover the Laws of the Celeftial Motions, and to meafure and 

 calculate thdm, but to account for their Caufe. 



Whether Sir Ifaac did fuppofe that the Motion of the Celeftial 

 Bodies was really begun by Impulfe, may perhaps be doubted ; 

 though he has certainly expreffed himfelf in fuch a way, as to give 

 reafon to believe that he really thought fo*. At the fame time, I 

 think it may be maintained, that this alfo was no more than hypo- 

 thecs ; for, as Motion, by Impulfe, is always in a Straight Line, 

 and as he was to analyfe the Planetary ?vIotion into two Straight 



Lines, 



move round fome one point, as a Centre, in the fame manner as one of our Planets 

 with its Satellites moves round the Sun. If the Uiiiverfe be aSyftem, as I think every 

 Theift muft believe it to be, it is a necefiary confequence that there muft be fame Prin- 

 ciple of union in it, and fome point, fuch as a Centre, to which all its Motions mult 

 have a relation. And it is alfo necefiary that there fhouid be fome conformity betwixt 

 the Motions of the feveral parts of the Sy ftem, and of the whole : And, therefore, as 

 v.e know but this our Solar Syftem, and that the Bodies in this Syflcm are moved 

 round a Centre, it is highly probable, that tlie Bodies in the other Syftems, and that 

 ihe Syfiems themselves, are moved in the fime way. But what has all this to do with 

 Gravitation, or that ftrange hypothefis rejeded by Sir Ifaac hidifelf in fuch flrong terms, 

 of Body operating upon Body at a diftance? 



* Whatever Sir Ifaac thoueht when when he wrote his Principia, I think it is evi- 

 (Jent from his Letters to Dr. Bentley, that when he wrote thtrm, he did not believe 

 that the Planets were fet in Motion by Impulfe of other Bodies, or by any natural 

 Caufe. See what I have faid, p, y^G, of the Progrefs of Sir Ifaac's Notions in this 

 Matter. 



