EMPIRICAL KNOWLEDGE, EXPLANATION, &c. 171 



results. The effects of even a few simple laws may be 

 infinitely diverse, and some of the most curious and 

 useful effects may remain undetected until accidental 

 observation brings them to our notice. 



Predicted Discoveries. 



The most interesting of the four classes of facts or 

 phenomena as specified in p. 157, is probably the third 

 containing those the occurrence of which has been first 



D 



predicted by theory, and then verified by observation. 

 There is no more convincing proof of the soundness of 

 scientific knowledge than that it thus confers the gift 

 of foresight. Auguste Comte said that ' Prevision is the 

 test of true theory ; ' I should say that it is only one test 

 of true theory, but that which is most likely to strike the 

 public attention. Coincidence with fact is the test of true 

 theory, but when the result of theory is announced before- 

 hand, there can be no possible doubt as to the unpre- 

 judiced and confident spirit in which the theorist inter- 

 prets the results of his own theory. 



The earliest instance of scientific prophecy is naturally 

 furnished by the science of Astronomy, which was the 

 earliest in development. Herodotus narrates 11 that, in 

 the midst of a battle between the Medes and Lydians, the 

 day was suddenly turned into night, and the event had 

 been foretold by Thales, the Father of Philosophy. A 

 cessation of the combat and a peace confirmed by mar- 

 riages was the immediate consequence of this happy 

 scientific effort. Much controversy has taken place con- 

 cerning the exact date of this occurrence, Baily assign- 

 ing the year 610 B.C., but Sir G. B. Airy has lately 

 decided that the exact day was the 28th of May, 584 B.C. 



n Lib. i. cap. 74. 



