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 mathematical proof, though no doubt cogent to the human mind (as at present constituted 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 assuming that the moon and planets move in ellipses, you can actually foretell things that happen, as for instance the occurrences of eclipses; and in reply to that I can only say that Tycho Brahé foretold eclipses almost as well by assuming that the heavenly bodies moved in epicycles, and that modern astronomers do apply the epicycle theory in their mathematical formulæ. 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; and as to the relative, why all we can say is that it does not exist (any more than species exists)--we cannot break up Nature so; it is not a thing in Nature, but in our own minds--it is a view and a fiction.[19]