182 JAMES A. WEISHEIPL 



such bodies have no active " nature " that they can, in the 

 scheme of Buridan, receive a perpetual impetus for continued 

 motion. This is quite different from Kilwardby's conception of 

 celestial spheres actively inclined to circular motion, for here 

 the " nature " of celestial bodies is an active principle. The 

 final result of both views may be similar or even identical, but 

 the theoretical foundation of Buridan's theory of impetus for 

 the heavens is profoundly dissimilar to the views of Kilwardby. 

 Kilwardby's view, however, was common enough in later 

 centuries. It was favored particularly by Platonists and semi- 

 Platonists. Notably Nicholas of Cusa attempted to explain the 

 circular motion of the heavens by an appeal to their orbicular 

 shape; their matter, being different from terrestrial matter, 

 naturally tended to move orbicularly, that is, by rotating.^^ 

 Copernicus himself explained the circular motion of the heav- 

 enly bodies by their spherical nature: 



Now we note that the motion of the heavenly bodies is circular. 

 Rotation is natural to a sphere and by that very act is its shape 

 expressed. For here we deal with the simplest kind of body, where- 

 in neither beginning nor end may be discerned, nor, if it rotates 

 ever in the same place, may the one be distinguished from the 

 other.^^ 



For Copernicus, as for Kilwardby before him, the substantial 

 form of a spherical body naturally tends to move spherically. 

 Surprisingly, for Copernicus the outermost sphere of the fixed 

 stars, though spherical by nature, was said to be at rest.^° It 

 must be admitted, however, that Copernicus was not concerned 

 with explaining the physical causes of celestial motion, as this 

 is beyond the scope of mathematical astronomy. 



We may seriously doubt that Kilwardby's reply influenced 

 later writers; it certainly did not influence John Buridan. 

 Nevertheless it does represent an important medieval view 

 concerning celestial motion. 



** Nicholas of Cusa, De ludo globi, lib. I (Basel, 1565), pp. 210-214. 

 ** N. Copernicus, De revolutionihus orbium caelestium, lib. I, c. 4 (Thorn, 1873) , 

 p. 14; also c. 8, pp. 21-24. 

 »" Ibid., c. 10, pp. 28-29. 



