GRAVITATIONAL MOTION 203 



introduce plurality and composition into circular motions, and 

 these too can be composed of many circular motions, of which each 

 is itself simple and one.^- 



The composition which Theodoric here attempts to explain 

 in terms of physical or natural causes is thus not the com- 

 position that would result from a straight-line gravitational 

 tendency to a center to which had been added a principle of 

 tangential motion, as this was to be proposed by Newton in 

 the seventeenth century, but rather a composition of rotary 

 motions consistent with the geometrical picture of the universe 

 already sketched by Ptolemy. What is interesting about Theo- 

 doric's view, however, is his willingness to account for the 

 deviations from perfect circularity detected by astronomers of 

 his time in planetary, lunar and solar motions, by allowing for 

 the possibility of different centers of gravity within the uni- 

 verse, and this while viewing these centers not merely mathe- 

 matically, but also as terms of proper natural motions from 

 intrinsic principles. This represents a very definite break with 

 the Averroist- Aristotelian tradition, and at the same time pro- 

 vides the basis for accomodating Aristotelian thought to a 

 plurality of gravitational centers, in the sense of universal 

 gravitation as it was ultimately to be understood by Newton. 



Another interesting development of Theodoric's thought 

 regarding composite motions is his attempt to explain the 

 complex motion of the tides in terms of natural principles. This 



^^ Cap. 17, U 59rb-va: Si inquam sic se habet in istis motibus rectis, ut dictum 

 est, quid mirabile vel inconveniens si sic vult intelligi Philosophus illud quod tractat 

 de motu circulari, dicens ipsum fieri circa medium, non sumendo medium omnino 

 pro centro universi, sed pro quocumque naturali medio cuiuscumque circuli naturalis 

 in quo invenitur eciam aliquis habens naturam termini, puta medium centrum 

 circa quod volvitur, que circumvolucio includit in se et importat naturam motus 

 ad terminum et a termino, quod utrumque intelligitur in respectum ad centrum 

 talis circuli. Moveri enim circa centrum est moveri quodammodo ab ipso et ad 

 ipsum; unde habet naturam et racionem termini motus. Si igitur habitudo diversa 

 ad diversos terminos motus rectos, ut visum est, sic eciam quoad motum circularem 

 secundum diversa media centralia, quorum quodlibet habet naturam et racionem 

 termini, potest plurificari et componi motus, ut sit motus compositus ex pluribus 

 circularibus motibus quorum quilibet in se simplex et unus est. . . . 



