150 JAMES A. WEISHEIPL 



and their movements, astronomy was recognized fully as hypo- 

 thetical. The true causes of celestial motion are extremely 

 difficult for any science to discover. " These matters into which 

 we inquire are difficult since we are able to perceive little 

 of their causes, and the properties of these bodies are more 

 remote from our understanding than the bodies themselves are 

 spatially distant from our eyes." ^^ Simplicius, and possibly 

 Plato before him, was aware that the aim of astronomy is to 

 give 3ome rational account of celestial phenomena, saving all 

 the known facts {'X(i)(,eLv ra ^aivoixeva) .^"^ But as it turns out, 

 all the known facts of astronomy can be explained by a variety 

 of hypotheses. Of course, when a new fact is discovered which 

 cannot be accomodated by the existing hypothesis, then some 

 new hypothesis must be devised to account for the new fact. 

 St. Thomas, commenting on the homocentric spheres of Plato 

 and Eudoxus, observes: 



The hypotheses which they devised (adinvenerunt) are not neces- 

 sarily true, for although the appearances are saved on the assump- 

 tion of those hypotheses, one does not have to say that they are 

 true, because the phenomena of celestial bodies may perhaps be 

 saved in some other way not yet known to man.^'^ 



An astronomical hypothesis which accounts for all the known 

 facts is indeed worthy of provisional credence. But every 

 astronomical hypothesis by its very nature was considered by 

 St. Thomas to be provisional and indemonstrative. Speaking 

 of this type of reasoning, St. Thomas notes: 



Reasoning is employed in another way, not as furnishing an 

 adequate proof of a principle, but as showing how the existing 

 facts are in harmony with a principle already posited; as in astron- 

 omy the theory of eccentrics and epicycles is considered as estab- 

 lished, because thereby the sensible appearances of celestial move- 

 ments can be explained; it is not, however, as if this proof were 



" St. Thomas, In II De caelo, lect. 17, n. 8. 



Cf. P. Duhem, " Sc^fetj/ ra ^aLPo/Meva. Essai sur la notion de theorie physique 

 de Platon a Galilee," Annales de philosophie chretienne (Paris), 4 serie, VI (1908), 

 113 ff., 277 flf., 352 ff, 482 ff., 561 ff. 



^' St. Thomas, In II De caelo, lect. 17, n. 2. 



