GRAVITATIONAL MOTION 197 



and this even when living things are excluded and one treats 

 only of objects that move precisely as light or heavy. This 

 leads Theodoric to a significant question: " Why do certain 

 composite bodies move naturally with a simple motion, and 

 certain others with a composite motion? " ^^ The answer he 

 proposes, while hardly consonant with modern scientific thought 

 on the subjects he treats, provides an insight into the way in 

 which the medievals explained such divergent motions as those 

 of currents, magnets, tides, and heavenly bodies, and may be 

 suggestive of analogous approaches available to the natural 

 philosopher of the twentieth century for evaluating modem 

 theories dealing with these same topics. 



Composite Motions 



In summary form, the general answer that Theodoric gives 

 to this question, which he then goes on to elaborate through 

 twelve chapters of the opusculum, is contained in the following 

 statement: 



It should be noted that there are many differences among bodies 

 that are moved by nature either with composite or simple motions. 

 Some are moved as parts of wholes, without being separate from 

 such wholes. Others are moved somewhat as wholes themselves, 

 and this in a twofold way, for some are moved by an intrinsic 

 natural principle, while others are moved by an extrinsic principle, 

 as will become apparent when we consider them singly .^^ 



To illustrate the meaning of this observation, we may note that 

 for Theodoric the natural motions of fluids, such as those com- 

 prising the atmosphere and the hydrosphere, are generally 

 composite motions. Some of these are composite in the sense 

 that they are motions of the parts of a fluid medium; the 

 movement of such a part he resolves into two interacting 



"Cap. 12, M 15ra, U 142ra-b. 



■^' Ibid.: Est sciendum quod corporum que moventur motu composite seu simplici 

 per naturam multiplex est differencia. Quedam enim moventur ut partes in toto, 

 non tamen separate a toto, quedam autem ut tota quedam, et hoc dupliclter, quia 

 quedam moventur ab intrinseco principio naturali, quedam ab extrinseco, ut de 

 singulis patebit. 



