Ixxxii REPORT — 1872. 



stop its motion, he takes as direct cognizance, through hia feeling of the Effort 

 required to resist it, of the force which produces that motion, as he does 

 through his eye of the motion itself. 



Now since it is universally admitted that our notion of the External World 

 would be not only incomplete, but erroneous, if our Visual perceptions were 

 not supplemejited 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 co-ordinate the idea of Force with that of Motion, and recognize it 

 as the " efficient cause " of those phenomena, — 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 Phi- 

 losophy 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 Dy- 

 namics. 



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 interpreta- 

 tion 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 

 shaU now, in the last place, endeavour to show you that it is the siibsti- 

 tution of the Dynamical for the mere Phenomenal idea, which gives their 

 highest value to our conceptions of that Order of Nature, which is wor- 

 shipped 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 differ- 

 ence 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 interpretation of that Motion given us by the profound in- 

 sight of Newton. Kepler's three Laws were nothing more than comprehen- 

 sive statements of certain groups of Phenomena determined by observation. 

 Thejirst, that of the revolution of the Planets in EUiptical 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 Eadius 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 gewralizations in which Kepler's are included ; to me they seem to 

 possess an altogether different character. For starting with the Conception 

 of two Forces, one of them tending to produce continiious uniform motion in 

 a straight line, the other tending to produce a uniformly accelerated motion 

 towards a fixed point, Newton's wonderful mastery of Geometrical reasoning 

 enabled him to show that, if these Dynamical assumptions be granted, 

 Kepler's p/ienomena^ "Laws," being necessary consequences of them, must 

 be universally true. And while that demonstration would have been alone 



