Chap. XV. ANTIENT METAPHYSICS. 183 



the propofitlan is true ; but I think it rauft be owned that it is ill ex-i 

 prefled *. 



Laying it down, therefore, as a certain fad, that all bodies we know 

 are moved^ or, at leaft, have a tendency to be moved ; and that, fo far 

 from there being a vis inertiae in them, there is in them a vis niobili^ 

 tcitis, let us next inquire into the feveral ways in which they are 

 moved. And it appears to me, that Ariftotle's divifion is full and 

 com pleat, viz. that they are moved, either in a ftraight line, a circu- 

 lar line, that is, a line revolving into itfelf, or in a direction mixed 



of 



* I have heard a better name given to this quality in body, viz. vis perfevermitiae^ 

 or, as we may exprefs it more {imply in Engliih, refijlcnce ; which undoubt- 

 edly is an eflentiai property in bodyj for, if body were altogether foft and yielding, 

 it would be incapable of either impelling or being impelled. The confequence of 

 •this would be, that we could have no fenfation, or perception of body by our fenfes, 

 which is produced by impulfe upon the organs of our fenfe, either immediately by the 

 body perceived, as in the cafe of Uuch and tajie^ or mediately, by the intervention of 

 other bodies, which are repelled by the body perceived, and, in that way, the motion 

 communicated to our organs, as in the cafe oi fight. And, further, there coulil be 

 no motion of body by impulfe of other body ; becauie, if there were no rcfiitence in body^ 

 there could be no impreflion made upon it. 



With refiftence are neceffariiy joined two other qualities of body, viz. foUdity and 

 impenetrability. By the firft of which is meant, inch a junction and coiietion of 

 parts, as neceffariiy make refifbence to any impreflion j and, by the other, the im- 

 polTibility that any two particles of matter fhoula occupy the fame fpace, which would 

 be the cafe, if one of them could fo penetrate the other, as perfectly to incorporate 

 with it. Thefe are qualites of matter, which we are fure, from obieivation and ex- 

 perience, that it poffefles : And therefore we may lay them down as the bafis of our 

 reafoning upon this fubjc£l, according to the rule of pnilofophiling above mentioned. 



1 have elfewhere in this work, (pag. 29- 30.) obferved an inaccuracy of exprclhon, 

 likewife, in another law of motion laid down by JSir ifaac Newton, that * a<fl:ion and 

 * re-aftion are equal.' 'I'hat I am right in the fenfe 1 have given to that axiom ot the 

 Newtonian philofophy, I will not pofitiveiy allirm. But this i think 1 may venture 

 to affirm, that it is not exprefled fo clearly .is an axiom ought to be cxpreneti : And, 

 in general,! cannot help obferving, that 8ir liaac, though h^. m.iy havt been as good, or 

 perhaps a better geometer, yet he was not fuch a mailer of language as iiuc'id, in 

 whofe elements there is a propriety and brevity of expreffion which 1 have often ad- 

 mired. 



