698 THE POPULAR SCIENCE MONTHLY. 



perceptions were not supplemented by our tactile, so, as it seems to 

 me, our interpretation of the phenomena of the Universe must be very 

 inadequate if we do not mentally coordinate the idea of force with 

 that of motion, and recognize it as the " efficient cause " of those phe- 

 nomena the " material conditions " constituting (to use the old 

 scholastic term) only " their formal cause." And I lay the greater 

 stress on this point, because the mechanical philosophy of the present 

 day tends more and more to express itself in terms of motion rather 

 than in terms of force to become kinetics, instead of 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 force is one of those elementary 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, endeavor 

 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 interpreters 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 dy- 

 namical conception that applies to them, is furnished by the contrast 

 between the so-called laws of planetary motion discovered by the per- 

 severing ingenuity of Kepler, and the interpretation of that motion 

 given us by the profound insight of Newton. Kepler's three laws 

 were nothing more than comprehensive statements of certain groups 

 of phenomena determined by observation. The first, that of the revo- 

 lution 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 eccentric 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 applicability 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 distances 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 were 

 merely higher generalizations in which Kepler's are inchided ; to me 

 they seem to possess an altogether different character. For, starting 

 with the conception of two forces, one of them tending to produce con- 

 tinuous uniform motion in a straight line, the other tending to produce 



