Chap.VIII. ANTIENT METAPHYSICS. 439 



Lines, of which the Motion in the Projcdile Line was the firft and 

 fundamental Motion, it was natural enough for him to lay down, by 

 way of hypothefis, that the Motion in that Line was begun by Im- 

 pulfe ; And, indeed, as the Projedile Line itfelf is a mere hypo- 

 thefis, we may, I think, with likelihood enough, fuppofc that the 

 beginning of Motion, by Lnpulfc, in that Line, is nothing elfe. 

 But, be that as it will, I think I can ^flcrt, Vv-ith fome confidence, 

 that his fyftem does not require that the Motion fliould be actually 

 fo produced, any more than it requires that the Planets ftiould be 

 ad:ually moved by a Force, either of Body or Mind, impelling thcni 

 towards the Centre. But it was neccffaty, for diicovcring the 

 Lav^rs of their Motions, and meafuring and computing them, to 

 analyfe them into two Motions, both in right Lines ; fo neceflary, 

 that I believe it was impoffible to do it ocherwife, as impofliblc 

 as it would have been for Euclid to have demonftrated his Propofi- 

 tions concerning Lines and Figures, without analyling Body into 

 Length, Breadth, and Depth, and confidering each of them fepara- 

 rately, though they can no more exift feparately than the Progreflive 

 Motion of the Planet and its defcent from the Tangent towards the 

 Centre; or as impoffible as it would be, in many cafes, to demonftrate 

 the Velocity by which a Body goes on in a Straight Line, by virtue 

 of a fmgle Impulfe by one Body, without refolving that fmgle Mo- 

 ving Power into two Moving Powers. And, in general, it may be 

 obferved, that no Science, whether of Body or of Mind, can be 

 treated accurately, without analyfing and feparating w^hat, by na- 

 ture, is infeparable; for, in the Sciences belonging to Mind, fuch 

 as Logic, Ethics, and Metaphyfics, it is as neceflary to proceed in 

 that manner, as in the Sciences concerning Body and its Motions. 



The Principles of Sir Ifaac's Aftronomy, as I have analyfed them, 

 appear to me fo fimple, that, like other great difcoveries, it feems 

 wonderful they were not fooner difcovered ; for, imo. It was ob- 

 vious 



