Chap. VII. ANTIENT METAPHYSICS. 431 



in time to come. And we may alfo carry this ratiocination to time 

 paft, and conclude that it has always been fo. It is in this way, 

 chiefly, that we reafon in natural philofophy; and it is our only me- 

 thod of reafoning concerning the affairs of life. I will begin with 

 our reafonings of this kind upon the lubjedt of Nature. 



It is by indudiori that we form all thofe general propofitions con- 

 cerning the motions and operations of body, which we call the laws 

 of motion and of nature ; for we can prove nothing concerning them 

 a priori^ not even that a ftone falls to the ground. As little can 

 we prove demonftratively, or a priori^ that the motion of fall- 

 ing bodies is uniformly accelerated ; a principle from which 

 many confequences are reducible, relating to the velocity of fall- 

 ing bodies, the fpace through which they fall, and the time : Nor 

 can we otherwife prove, though I believe the general opinion is dif- 

 ferent, that grand principle of mechanics, *That the moment of every 

 ' bc'dy IS in a compound ratio of the mafs of its matter, and its veloci- 



* ty ;' for by no reafoning a priori can it ever be demonftrated, that as 

 much as I take away from the mafs of any body, if fo much I add ta 

 its velocity, the moment or 'weight will continue the fame. — In the fame 

 manner, we know, * That, if one body, by impulle, moves another, 

 ' as much as the body moved acquires of the motive force, fo much 



* will the body moving lofe' : Or, as the Newtonians chufe to cxprefs 

 it, Aciion and re-aftion are equal : Andin this way, and in no other, is 

 it proved, that body, moved by impuhe of another body, goes on in 

 the dircdion of a flraight line, and retains the motion coma.unicated 

 to it till It be retarded, or flopped alroirethcr, by (ome contrary 

 force. Thefe, and other propofitions concerning motion, being found 

 always to hold in every indance in which they have been tried, from 

 them general conclufions are fbrnsed, whi.h we call hnvs of nature • 

 and, confidering them as axioms or felf-evident propofitions in natural 

 philofophy, we reafon downward from thein, and in that way 

 ftriaiy ocmonftrate or prove a priori^ that is, from cuu/e to effe^. 



And 



