180 JAMES A. WEISHEIPL 



East to West, and still others move naturally as epicycles, and 

 others on the eccentric. To each planet and orb God gave an 

 innate natural inclination to move in a particular way in rota- 

 tional motion; to each He accorded an innate order, regularity 

 and direction without the need of a distinct agency like a soul, 

 an angel or Himself here and now producing the motion. " Just 

 as the forces (pondera) of heavy and light move bodies con- 

 sistently, not permitting them to stray outside a determined 

 path, so it is with the forces of each and every celestial body." 

 Consequently rotational motion is as natural to celestial bodies 

 as gravitational motion is to heavy bodies. Both arise spon- 

 taneously from nature as an intrinsic active principle, instinctu 

 proprioruTn ponderum (q. 3) . It was commonly recognized 

 among the schoolmen that heavy bodies need nothing more 

 than their own generated nature to account for gravitational 

 motion; heavy bodies need no conjoined mover to account for 

 the continued downward fall.®^ Kilwardby wished to explain 

 celestial motions by a similar intrinsic formal principle. Ter- 

 restrial bodies are unattached and hence move rectilinearly to 

 a place of relative rest. But for Kilwardby the heavens are 

 spherical; stars and planets are attached to their proper orbs 

 within a sphere. Consequently the only " natural " motion the 

 heavens could have is rotational, a continual rotation of each 

 orb on its axis. The combination of various rotations on suit- 

 able axes together with the required uniform velocity of each 

 rotation produced the apparent motion of the planet. Kil- 

 wardby thus dispenses with the need of any conjoined or 

 separated mover, whether that mover be called a soul, an 

 angel, intelligence or God. It is clear from this that Kilwardby 

 could not prove the existence of God through physical motion. 

 He cannot even prove the existence of a separated substance. 

 Because of the great diversity of opinion concerning celestial 

 movers, Kilwardby maintained that it is impossible to prove 

 that angels move the spheres (q. 4) . Philosophers think that 

 they have infallibly demonstrated the existence of spiritual 



*^ Cf. J. A. Weisheipl, Nature and Gravitation, ed. cit., pp. 19-21, 25-28. 



