534 DISSERTATION ON 



going by a momentary impulfe, it will go on forever in a certain di- 

 redion, and with the fame degree of velocity. And, indeed, if it were 

 an axiom, it would be very extraordinary, that no philofopher ever 

 thought of it before Des Cartes, w^ho firft devifed it, to fupport his 

 mechanical philofophy ; and for the fatne reafon it was adopted by 

 Sir Ifaac Newton. 



If, therefore, it be not an axiom, and yet be true, it muft be 

 proved. This can be done only in two ways ; either a priori, that 

 is, from the na-ture of the thing, or a pojleriorh that is, from fadl and 

 experiment. Now, in this latter way, it is impoflibleit can be proved : 

 For, I wo, Powers and Faculties are latent things, which may be appre- 

 hended by intellect, but cannot be perceived by fenfe ; fo that it is im- 

 poflible, by any experiment or obfervation, to difcover by what power 

 a body is moved by itfelf, when the operation of another body upon it 

 has ceafed. And, zdo. Though the fad be true, that a body continues 

 in motion after the impulfe which put it in motion has ceafed, yet, 

 we find that all the motions fo produced, do languifli and decay by 

 degrees, and, at laft, flop altogether. This, I know, the Newtonians 

 impute to fome obftacle in the medium, which cannot be entirely re- 

 moved by any human art ; but they tell qs, that the rarer they can 

 make the medium, the longer will the motion continue ; and from 

 thence they conclude, that, if they could make a perfed vacuum, it 

 would continue for ever ; but this conclufion is far from being de- 

 monftrative ; for it fuppofes the very thing in queftion, namely, that 

 the motion is, by its nature, perpetual, and therefore can only be 

 put an end to by fomething external; whereas, I fay, that the motion 

 is not, by its nature, perpetual j and therefore may ceafe in two ways, 

 either by the refiftence of obftacles, or by decaying and perifhing 

 through time, as every other thing in this fublunary world does. 



It feems, therefore, evident, that the propofition cannot be proved 

 apojl^riori'y and, if fo, it only remains to be confidered whether it dan 



be 



