206 W. A. WALLACE 



posite motion, one being the moon as efficient cause, the other 

 being the passive nature of sea water, which is capable of 

 receiving the moon's influence because of its peculiar material 

 composition. 



Extrinsic Movers 



Having thus accounted for several composite natural motions, 

 Theodoric turns to a question which was much agitated by 

 medieval scientists, and whose resolution gTadually prepared 

 the way for the new mechanics that was to arise with Galileo 

 and Newton. This was the problem of identifying the extrinsic 

 mover that is responsible for falling motion, i. e., gravitational 

 movement to a center. Theodoric has referred previously to a 

 " generative principle " as accounting for the composite charac- 

 ter of some natural motions; this resembles the traditional 

 Aristotelian doctrine that the generator is the per se cause of 

 simple natural motions, and thus the question arises whether 

 Theodoric also regards the generator as being the effective 

 principle that moves a body falling in straight-line gravitational 

 motion. Theodoric's answer to this question is negative. While 

 developed in the context of Aristotle's natural philosophy, his 

 solution is again representative of a transitional type of rea- 

 soning that in some ways anticipates the development of 

 sixteenth-century mechanics, and on this account, at least, is 

 worthy of note.^^ 



Theodoric's line of argumentation is directed principally 

 against those who interpret Aristotle to mean that gravitating 

 bodies are moved by the generator in the sense that they have 

 their form and species from the generator, and just as they 

 have these, so " they have all the natural accidents which 

 follow from the species, one of which is natural motion with 

 respect to place." *° Such was not an uncommon interpretation 



secundum aliam sui partem que in loco continuo ad presenciam lune vaporat et 

 extenditur et fluit, sicut dictum est. . . . 



^^ See Maier, " Das Problem der Gravitation," Studien III, pp. 143-254. 



*°Cap. 28, M 16vb-17ra, T I84r, U 144va: Dicunt autem quidam quod gravia et 

 levia et universal iter ea que moventur localiter per naturam in hoc inferiori mundo 



