THE NEWTONIAN PHILOSOPHY. 531 



the firft part of this law, which fays that body at reft continues at reft 

 till it be moved by fome power different from itfelf ; or, in other 

 words, that body cannot move itfelf; it is a propofition that every phi- 

 lofopher, who is not a materialift, muft affent to. And it is equally 

 certain, that body, once put in motion, cannot, of itfelf, alter the di- 

 rection of that motion, nor make it either quicker or flower. The 

 only queftion, therefore, is, Whether a body, once put in motion, can, 

 of itfelf, and by its own natural power, continue the motion ? I fay, 



* of itfelf, and by the power of its own nature ;* for that undoubtedly 

 is the meaning of Sir Ifaac's i>is inftta, which Sir Ifaac, in his third de- 

 finition, applies equally to that power by which a body at reft conti- 

 nues at reft, and that power by which, being in motion, it continues 

 in motion : And he makes it a general property of all matter ; for he 

 fays, * Materiae vis inftta eft potentia reftjlendi^ qua corpus unumquod- 



* que^ quantum infe ejiy pctjeverat injlatu fuo vel quiefcendi vel mo- 



* vendi uniformiter in dire6lum,^ Now, that all matter, by its own 

 nature and efl*ence, and by that perfed paffivity which all theifts 

 afcribe to it, and by which they eflentially diftinguifli it from Mind, 

 continues at reft when it is at reft, is evident ; and, by the fame 

 power and neceflity of its nature, according to Sir Ifaac, it continues 

 in motion when it is in motion. What makes Sir Ifaac's meaning ftill 

 further evident, is the diftinaion that he makes in this third defini- 

 tion, and in the following, betwixt the vis inftta and the vis impreJJa^ 

 by which laft he undoubtedly means an external force imprefled up- 

 on the body, in oppofition to that internal power eflential to its na- 

 ture, by which it continues either at reft or motion *. 



It is to be remembered, that all Sir Ifaac*s definitions concerning 

 motion relate only to motion produced by the operation of body upon 



X '^ 2 body. 



* I obfervc that the vis infita is tranflated by Dr John Clarke, in what he has 

 written upon Sir Ifaac's principles of philofophy, inherent forte. 



