412 ANTIENT METAPHYSICS. Book V. 



But what way arc we to eftimate this Power or Force, by which a^ 

 planet is carried on in an orbit of certain dinienfions in a given 

 time ? And here I think it is not at all neceflliry to inquire A^hat 

 the power is that carries on the Planet in its Motion, whether Body 

 or Mind, or how that power operates, whether by Pulfion or Tru- 

 fion, as Body operates upon Body, or whether by Animation, as 

 Mind moves Body. But all I want to know, and all, I think, that 

 is neceflliry to be known for the purpofe of Aftronomy, is the quan- 

 tity of the Moving Force. And this appears to me to have been 

 the notion that Sir Ifaac had of the Science : For he fays that he 

 does not inquire concerning the Species Viriurn^ and the ^alitates 

 Ph^ficas ; fed ^lantitates et Proportioncs Mathemat'tcas expendlt. 

 And in this way I think Aftronomy is properly dlftinguiflicd, both 

 from Metaphyfics and Phyfics : For it is the bufinefs of Metaphyfics 

 to inquire concerning the Caufes of every thing, and particularly 

 the Caufe of Motion ; and it is the bufinefs of Phyfics to inquire into 

 the qualities of Bodies, confidered merely as Bodies, and to difcri- 

 minate them accurately from one another ; and, with refpedl to their 

 Motions, only to difcover what efFeds thefe produce upon the qua- 

 lities of the Bodies, but not to meafure, or calculate or inveftigate tlie 

 Laws of thefe Motions, which belongs either to Mechanics, with 

 refpeft to the Motions here on Earth, or to Aftronomy, with refpedt 

 to the Motions of the Celeftial Bodies. 



Underftanding, therefore, that it is the proper bufinefs of Aftro- 

 nomy, confidered as a Science, to inveftigate the force of the Mov- 

 ing Power by which the Planets are carried on in their orbits, I 

 proceed to inquire how that Force is to be inveftigated. As Powers 

 and Forces are, by their nature, latent things, we can only eftimate 

 the Power which moves any Body by the eifeft that it produces : 

 And this effeft is no other than the velocity wath which the Body is 

 moved ; fo that, if we can difcover any rule or ftandard by which 

 7 we 



