434 HISTORY OF FORMAL ASTRONOMY. 



obtain each truth ; every accepted doctrine must 

 have been one selected out of many candidates. In 

 making many conjectures, which on trial proved 

 erroneous, Kepler was no more fanciful or unphilo- 

 sophical than other discoverers have been. Dis- 

 covery is not a " cautious" or "rigorous" process, in 

 the sense of abstaining from such suppositions. But 

 there are great differences in different cases, in the 

 facility with which guesses are proved to be errours, 

 and in the degree of attention with which the 

 errour and the proof are afterwards dwelt on. 

 Kepler certainly was remarkable for the labour 

 which he gave to such self-refutations, and for the 

 candour and copiousness with which he narrated 

 them ; his works are in this way extremely curious 

 and amusing ; and are a very instructive exhibition 

 of the mental process of discovery. But in this 

 .respect, I venture to believe, they exhibit to us the 

 usual process (somewhat caricatured) of inventive 

 minds : they rather exemplify the rule of genius 

 than (as has generally been hitherto taught,) the 

 exception. We may add, that if many of Kepler's 

 guesses now appear fanciful and absurd, because 

 time and observation have refuted them, others, 

 which were at the time equally gratuitous, have 

 been confirmed by succeeding discoveries in a man- 

 ner which makes them appear marvellously saga- 

 cious ; as, for instance, his assertion of the rotation 

 of the sun on his axis, before the invention of the 

 telescope, and his opinion that the obliquity of tho 



