THE ANATOMY OF SCIENCE 73 



Lacking grasp of the idea of progress, the Greeks (with such nota- 

 ble exceptions as Aristarchus) showed comparatively little willing- 

 ness to consider theoretical postulates that could not be made to seem 

 self-evident. Such willingness represents a comparatively recent cast 

 of mind. It emerges very gradually—it really struggles forth— in the 

 16th and 17th centuries. The multiple motions of the earth postulated 

 by Copernicus are no self-evidencies, but Copernicus still feels bound 

 to justify them by placing a strong emphasis on the self-evidency of 

 other parts of his system: e.g., the perfect circular motions of celes- 

 tial bodies, troubled by no such shabby evasion as the equant; the 

 appropriately central position of the sun, the majestic giver of heat 

 and light to our world. Almost a century later Descartes still thinks 

 to construct a theoretic system by selecting as postulates only those 

 to him both clear and indubitable, but in this Descartes' is an atti- 

 tude more ancient than modern. What was to become the modern 

 attitude is most vividly expressed in Newton's system. Here the 

 postulates (e.g., of a universal gravitational attraction propagated 

 through space) were defended on no appeal to this or that self- 

 evidency. They were defended quite forthrightly on a newly funda- 

 mental plane: they work. Thus we are taught a more modest inter- 

 pretation of the principle of intelligibility. We learn to content our- 

 selves with explanations in terms of theoretical postulates not neces- 

 sarily self-evident, and so gain power to work with a far greater range 

 of possibilities than before. Believing in scientific progress, we are 

 confident that— if never "self-evident"— our postulates will some day 

 be explained, though then only in terms of still more fundamental 

 postulates themselves tmexplained. 



A SECOND QUALIFICATION OF THE PRINCIPLE 

 OF INTELLIGIBILITY 



The principle of intelligibility may lead men to the inordinately 

 optimistic expectation that all aspects of experience— physical, bio- 

 logical, aesthetic, mystical— can be promptly rationalized within one 

 immense system. Thus Aristotle seeks to build a final system, in ap- 

 parently complete confidence that the fullest understanding achiev- 

 able by man can at one stroke be won. Thus Epicurus' atomism is for 

 him not only a comprehensive physical theory but also a cosmology 

 and an ethic. Not all of the ancients pursued such grandiose aims: 

 in such specialized works as the geometry of Euclid, the statics of 



