242 A Short History of Astronomy [CH. ix. 



greatest genius that ever existed, and the most fortunate, 

 for we cannot find more than once a system of the world 

 to establish," shew the immense respect for his work felt 

 by those who were most competent to judge it. 



With these magnificent eulogies it is pleasant to compare 

 Newton's own grateful recognition of his predecessors, 

 " If I have seen further than other men, it is because I 

 have stood upon the shoulders of the giants," and his 

 modest estimate of his own performances : 



"I do not know what I may appear to the world ; but to 

 myself I seem to have been only like a boy playing on the sea- 

 shore, and diverting myself in now and then finding a smoother 

 pebble or a prettier shell than ordinary, whilst the great ocean 

 of truth lay all undiscovered before me." 



194. It is sometimes said, in explanation of the differ- 

 ence between Newton's achievements and those of earlier 

 astronomers, that whereas they discovered how the celestial 

 bodies moved, he shewed why the motions were as they 

 were, or, in other words, that they described motions while 

 he explained them or ascertained their cause. It is, 

 however, doubtful whether this distinction between How 

 and Why, though undoubtedly to some extent convenient, 

 has any real validity. Ptolemy, for example, represented 

 the motion of a planet by a certain combination of epi- 

 cycles ; his scheme was equivalent to a particular method 

 of describing the motion; but if any one had asked him 

 why the planet would be in a particular position at a 

 particular time, he might legitimately have answered that 

 it was so because the planet was connected with this par- 

 ticular system of epicycles, and its place could be deduced 

 from them by a rigorous process of calculation. But if 

 any one had gone further and asked why the planet's 

 epicycles were as they were, Ptolemy could have given no 

 answer. Moreover, as the system of epicycles differed in 

 some important respects from planet to planet, Ptolemy's 

 system left unanswered a number of questions which 

 obviously presented themselves. Then Coppernicus gave 

 a partial answer to some of these questions. To the 

 question why certain of the planetary motions, correspond- 

 ing to certain epicycles, existed, he would have replied that 

 it was because of certain motions cf the earth, from which 



