2o6 NATURE AND MAN. 



more to express itself in terms of motion rather than in terms of 

 force ; — to become kinetics instead oi dynamics. 



Thus from whatever side we look at this question, — whether 

 the common sense of mankind, the logical analysis of the relation 

 between cause and effect, or the study of the working of our own 

 intellects in the interpretation of Nature, — we seem led to the 

 same conclusion; that the notion of y^;-^!? is one of those ele- 

 mentary forms of thought with which we can no more dispense, 

 than we can with the notion of space or of succession. And I 

 shall now, in the last place, endeavour to show you that it is the 

 substitution of the dynamical for the mere phenomenal idea, which 

 gives their highest value to our conceptions of that order of 

 Nature, which is worshipped as itself a god by the class of inter- 

 preters whose doctrine I call in question. 



The most illustrative as well as the most illustrious example 

 of the difference between the mere generalization of phenomena 

 and the dynamical conception that applies to them, is furnished 

 by the contrast between the so-called laws of planetary motion 

 discovered by the persevering ingenuity of Kepler, and the inter- 

 pretation of that motion given us by the profound insight of 

 Newton. Kepler's three laws were nothing more than compre- 

 hensive statements of certain groups of phenomena determined 

 by observation. The first, that of the revolution of the planets 

 in elliptical orbits, was based on the study of the observed 

 places of Mars alone ; it might or might not be true of the other 

 planets ; for, so far as Kepler knew, there was no reason why 

 the orbits of some of them might not be the excentric circles 

 which he had first supposed that of Mars to be. So Kepler's 

 second law of the passage of the radius vector over equal areas in 

 equal times, so long as it was simply a generalization of facts in 

 the case of that one planet, carried with it no reason for its ap- 

 plicability to other cases, except that which it might derive from 

 his erroneous conception of a whirling force. And his third law 

 was in like manner simply an expression of a certain harmonic 

 relation which he had discovered between the times and the dis- 

 tances of the planets, having no more rational value than any 

 other of his numerous hypotheses. 



Now the Newtonian "laws" are often spoken of as if they 



