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A CRITICISM. 



e might then call the path an irregular curve somewhat resembling an 

 ellipse. This is a new view. But on further enquiry it appears that, 

 while the moon is going round the earth, the earth itself is speeding on 

 through space about the sun in consequence of which the actual path 

 of the moon does not in the least resemble an ellipse ! Finally the sun 

 itself is in motion with regard to the fixed stars, and they are in move- 

 ment too. What then is the path of the moon ? No one knows; we 

 have not the faintest idea the word itself ceases to have any assignable 

 meaning. It is true that if we agree to ignore the perturbations pro- 

 duced by the sun as in fact we do ignore perturbations produced by the 

 planets and other bodies and if we agree to ignore the motion of the 

 earth, and the flight of the solar system through space, and even the 

 movement of any centre round which that may be speeding, we may 

 then say that the moon moves in an ellipse. But this has obviously 

 nothing to do with actual facts. The moon does not move in an ellipse 

 not even "relatively to the earth " and probably never has done and 

 never will do so. It may be a convenient view or fiction to say that it 

 would do so under such and such circumstances but it is still only a 

 fiction. To attempt to isolate a small portion of the phenomena from 

 the rest in a universe of which the unity is one of Science's most cherished 

 convictions, is obviously self-stultifying and useless. 



But you say it can be proved by mathematics that the ellipse would 

 be the path under these conditions; to which I reply that the mathemati- 

 cal proof, though no doubt cogent to the human mind (as at present con- 

 stituted in most people), is open to the same objection that it does not 

 deal with actual facts. It deals with a mental supposition, i. e., that 

 there are only two bodies acting on each other a case which never has 

 occurred and never can occur and then, assuming the law of gravitation 

 (which is just the thing which has to be proved), it arrives at a mental 

 formula, the ellipse. But to argue from this process that the ellipse is 

 really a thing in Nature, and that the heavenly bodies do move or even 

 tend to move in ellipses, is obviously a most unwarrantable leap in the 

 dark. Finally you argue that the leap is warranted because, by assum- 

 ing that the moon and planets move in ellipses, you can actually foretell 

 things that happen, as for instance the occurrence of eclipses; and in 

 reply to that I can only say that Tycho Brahe foretold eclipses almost as 

 well by assuming that the heavenly bodies moved in epicycles, and that 

 modern astronomers actually do apply the epicycle theory in their math- 

 ematical formulae. The epicycles were an assumption made for a certain 

 purpose, and the ellipses are an assumption made for the same purpose. 

 In some respects the ellipse is a more convenient fiction than the epicycle, 

 but it is no less a fiction. 



In other words with regard to this ' ' path of the moon " (as with 

 regard to any other phenomenon of Nature) our knowledge of it must 

 be either absolute or relative. But we cannot know the absolute path; 



