Chap. VI. ANTIENT METAPHYSICS. 411 



Motion of the Planets, from which all the other properties could be 

 demonftrated, in the fame manner as in Geometry there would be 

 no fclence of Triangles, if all the properties of them could not be 

 deduced demonftratively from that property which conftitutes the 

 nature and elTence of a Triangle, nauiely, its being bounded by 

 three lines. 



But how is this eflential quality of the Planetary Motion to be 

 difcovered, from which the phenomenon found out by Kepler, and 

 all the other phaenomena of the Motion of the Planets, are to be de- 

 duced ? Before we enter upon this inveftigation, I think we fhould 

 firft confider what conftitutes the nature and eflence of every Mo- 

 lion : And this, I think, muft be two things ; firft, the Motive 

 Force or Power, by which the Motion is carried on ; and, fecondly, 

 the diredion of the Motion. If thefe two are known, I think 

 the nature and eflence of the Motion muft neceflarily be known. 

 Now, one of thefe, I mean the diredlion of the Motion, is a matter 

 of fa(5t and obfervation ; for in that way it is kriown that the 

 Planets move in Ellipfes round their feveral centres, or, to fpeak 

 more properly, the Focus of the Ellipfe, in which the Sun, or Pri- 

 mary Planet, is placed : And, as the greater and lefler diameters of 

 thefe Ellipfes are likewife known, and, by confequence, the nature 

 of the Ellipfis, whether it be very oblong, like that of the Comets, 

 or coming nearer to a Circle, like thofe of the Planets, it follows, 

 by neceflary confequence, that the direftion of the Motion, that is, 

 the Curvlture of the Orbit, muft likewife be known : And fo far 

 we know alfo with refpeft to the Moving Force, that we know the 

 period of the Motion. Without thefe fafts being known, it would be 

 impoflible that we could find out what we are fecking ; for I agree 

 with the Newtonians, and all the modern philofophers, that we can 

 know nothing of Nature but from fadls and obfervations, and that 

 the philoiophy of Nature is nothing elfe but dcdudions, which Sci- 

 ence makes from thefe fads. 



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