244 d Short History of Astronomy [CH. ix. 



planets are of certain sizes, at certain distances from the 

 sun, etc., and to these questions again Newton could give 

 no answer. 



But whereas the questions left unanswered by Ptolemy, 

 Coppernicus, and Kepler were in whole or in part answered 

 by their successors, that is, their unexplained facts or 

 laws were shewn to be necessary consequences of other 

 simpler and more general laws, it happens that up to the 

 present day no one has been able to answer, in any satis- 

 factory way, these questions which Newton left unanswered. 

 In this particular direction, therefore, Newton's laws mark 

 the boundary of our present knowledge. But if any one 

 were to succeed this year or next in shewing gravitation to 

 be a consequence of some still more general law, this new 

 law would still bring with it a new Why. 



If, however, Newton's laws cannot be regarded as an 

 ultimate explanation of the phenomena of the solar system, 

 except in the historic sense that they have not yet been 

 shewn to depend on other more fundamental laws, their 

 success in " explaining," with fair accuracy, such an immense 

 mass of observed results in all parts of the solar system, 

 and their universal character, gave a powerful impetus to 

 the idea of accounting for observed facts in other depart- 

 ments of science, such as chemistry and physics, in some 

 similar way as the consequence of forces acting between 

 bodies, and hence to the conception of the material universe 

 as made up of a certain number of bodies, each acting on 

 one another with definite forces in /such a way that all the 

 changes which can be observed to go on are necessary 

 consequences of these forces, and are capable of prediction 

 by any one who has sufficient knowledge of the forces and 

 sufficient mathematical skill to develop their consequences. 



Whether this conception of the material universe is 

 adequate or not, it has undoubtedly exercised a very 

 important influence on scientific discovery as well as on 

 philosophical thought, and although it was never formulated 

 by Newton, and parts of it would probably have been 

 repudiated by him, there are indications that some such 

 ideas were in his head, and those who held the conception 

 most firmly undoubtedly derived their ideas directly or 

 indirectly from him. 



