iQ 4 ] Explanation and Description 243 



these (apparent) planetary motions could be de'duced as 

 necessary consequences. But the same information could 

 also have been given as a mere descriptive statement that 

 the earth moves in certain ways and the planets move in 

 certain other ways. But again, if Coppernicus had been 

 asked why the earth rotated on its axis, or why the planets 

 revolved round the sun, he could have given no answer; 

 still less could he have said why the planets had certain 

 irregularities in their motions, represented by his epicycles. 



Kepler again described the same motions very much 

 more simply and shortly by means of his three laws of 

 planetary motion ; but if any one had asked why a planet's 

 motion varied in certain ways, he might have replied th..i 

 it was because all planets moved in ellipses so as to sweep 

 out equal areas in equal times. Why this was so Kepler 

 was unable to say, though he spent much time in specu- 

 lating on the subject. This question was, however, answered 

 by Newton, who shewed that the planetary motions were 

 necessary consequences of his law of gravitation and his 

 laws of motion. Moreover from these same laws, which 

 were extremely simple in statement and few in number, 

 followed as necessary consequences the motion of the 

 moon and many other astronomical phenomena, and also 

 certain familiar terrestrial phenomena, such as the behaviour 

 of falling bodies ; so that a large number of groups of 

 observed facts, which had hitherto been disconnected from 

 one another, were here brought into connection as neces- 

 sary consequences of certain fundamental laws. But again 

 Newton's view of the solar system might equally well be 

 put as a mere descriptive statement that the planets, etc.,, 

 move with accelerations of certain magnitudes towards one 

 mother. As, however, the actual position or rate of motion 

 of a planet at any time can only be deduced by an extremely 

 elaborate calculation from Newton's laws, they are not at 

 all obviously equivalent to the observed celestial motions, 

 and we do not therefore at all easily think of them as being 

 merely a description. 



Again Newton's laws at once suggest the question why 

 bodies attract one another in this particular way ; and this 

 question, which Newton fully recognised as legitimate, he 

 was unable to answer. Or again we might ask why the 



