Chap. XIX. ANTIENT METAPHYSICS. 263 



motions fo contrary. Even if we were to fuppofe that a principle 

 of movement was effential to the body, or, in other words, that the 

 body moved itfelf, how could we conceive that it fhould move itfelf 

 two oppofite ways at once, or, that there fliould be two principles 



of movement in the fame body counteracting one another. In 



fhort, if the univerfe be governed by one principle, whether fiiind 

 or matter, it appears impoITible that the motions of the celeftial bo- 

 dies {hould be mixed t)f fuch different motions, and that, therefore, 

 Sir Ifaac's fyftem cannot be defended, except upon the hypothecs of 

 two principles in the univerfe controlling and counteracting one ano- 

 ther ; an hypothefis which I am far from thinking Sir Ifaac meant to 

 maintain. 



I have never heard any thing fald againft this reafoning, except 

 that the nature of the circular or elliptical motion is fuch, that, fup- 

 pofing no previous tendency to the center, yet it cannot be performed 

 without both that tendency, and a tendency to fly off in a tangent, or 

 ftraight line ; fo that it is neceffarily a compofed motion of a centn- 

 petal and centrifugal force. 



If this propofition can be demonftrated, I muft no doubt fubmit to 

 it. This, however, is not pretended ; but I have been told, that the 

 evidence of it is fuperior to any demonftration ; for it is an axiom. 

 But this I venture abfolutcly to deny ; for, though 1 do not pretend to 

 have learned any more than the elements of geometry, axioms and 

 definitions belong to the elements. Now, I have learned from my 

 mafters, the antients, in geometry, as well as every other fcience, that 

 the circle or ellipfis is the moft fimple of all figures, being defcribcd 

 by one line revolving into itfelf; whereas, every other figure mufl: be 

 formed by more than one line, and, if it is a rectilineal figure, 

 by three lines at lealt ; confcquently, the circular or elliptical mo- 

 tion, muft, of all motions delcribing figure, that is, inclofing fpace, 



be 



