Chap. I. A N T I E N T METAPHYSICS. 321 



ever in a ftraight line, when once begun. — This firfl: axiom, I muft 

 fuppofe, applies to the Motion of the Planets : And, indeed, Sir Ifaac 

 fo applies it himfelf, in his oblervations upon it ; nor could it other- 

 wife have flood, with any propriety, at the head of his fyftem of 

 Aftronomy. Now, as I prefume that Sir Ifaac knew fo much of the 

 nature of Mind, as to believe it to be an immaterial fubftance, I can- 

 not fuppofe that he would have ufed the words Vis Imprcjfa or hn- 

 pulfus^ to exprefs the Aclion of Mind upon Body ; for it is impof- 

 fible to conceive that an Immaterial Subftance can adl upon Body by 

 Pulfion, Trufion, PrelTure, or, in Ihort, in any way in which Body 

 ads upon Body *. I therefore underftand this firft axiom to relate to 

 Motion produced by Bodily Impulfe only, fuch as is the Motion of 

 Proje£tiles here on earth ; but I do not underftand it, for the reafons 

 above mentioned, to be confined to the Motion of Projedtiles, but to 

 be underftood likewife of the Motion of the Planets, which, there- 

 fore, Sir Ifaac fuppofes to be produced by Bodily impulfe, as Projec- 

 tiles here are moved. 



2^0, Astother/j Centripeta^ which produces the Motion of Gravi- 

 tation, if It be admitted that the Vis Imprejfa is Bodily Impulfe, I think 

 it is impofllble to deny that the Vis Centripeta is fo likewife, in the 

 language of Sir Ifaac ; for, in his fourth definition, he confiders it as 

 a fpecies of the Vis Imprejfa : His words are, ' Eft autem Vis Imprejfa 

 * diverfarum Originum, ut ex I6iu^ ex Prejlone, ex Vi Centripeta ;' 

 where we may obferve that he joins it with I^us and Prejfio^ by 

 •which words he never could have meant to exprefs the adtion of 

 Mind upon Body. And, in his fifth definition, he applies to it the 

 words dranving and impelling^ which can only be underftood of the 

 adion of Body upon Body. In the demonftration above mentioned, 

 of the firft propofition of the fecond fedion of his firft book, he is 

 Vol. II. S s more 



• See this explained at more length, page 47. 



