NATURE AND LAW. 379 



move in elliptical orbits, and that, if these orbits can be exactly 

 determined by observation, and the influence of perturbing forces 

 rightly estimated, their return can be predicted, may be considered 

 as fully justifying such an expectation, so far at least as the solar 

 system is concerned. 



But the "law" at which we thus arrive, is only a higher and 

 more comprehensive generalization of the facts of celestial observa- 

 tion, and rests on assumptions which are not only improved but 

 unprovable. For the idea of continuous onward motion in a 

 straight line, as the result of an original impulsive force not 

 antagonized or affected by any other — formularized by Newton 

 as his first " law of motion " — is not borne out by any acquired 

 experience, and does not seem likely to be ever thus verified. 

 For in no experiment we have it in our power to make, can we 

 entirely eliminate the antagonizing effect of friction and atmo- 

 spheric resistance ; and thus all movement that is subject to this 

 retardation, and is not sustained by any fresh action of the im- 

 pelling force, must come to an end. Hence the conviction com- 

 monly entertained that Newton's first " law " of motion must be 

 true, cannot be philosophically admitted to be anything more 

 than a high probability, based on the fact that the more completely 

 we can eliminate all antagonizing influences, the nearer we get to 

 the perpetuity of movement once initiated. To say that this " law " 

 is so self-evident that we cannot help accepting it as an " axiom " or 

 necessary form of thought, is to run counter to the historical fact, 

 that the great thinkers of antiquity— whom none have ever sur- 

 passed in pure thinking power — accepted as the dictate of universal 

 experience, that all terrestrial motions come to an end ; and were 

 thus led to range the celestial motions in a different category, as 

 going on for ever. 



So, again, we have no proof, and in the nature of things can 

 never get one, of the assumption of the attractive force exerted 

 either by the earth, or by any of the bodies of the solar system, 

 upon other bodies at a distance.* All that we can be said to 



* Newton himself strongly felt that the impossibility of rationally accounting 

 for action at a distance through an intervening vacuum, was the weak point of 

 his system. The science of the present day .is seeking for the solution of this 

 difficulty, in the hypothesis of the universal pervasion of space by moving mole- 

 cules of some form of highly attenuated matter. 



I"7 



