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Copernicus expounded in the true system of the universe; 
how this system received further exposition by the three - 
phenomenal laws discovered by Kepler ; and how these em- 
pirical laws were exhibited by Newton as necessary results of 
a universal law of gravitation, are facts too well known to 
require more than the briefest notice. The assumption of the 
very simple law that the force of gravity is proportional to 
the product of the gravitating masses directly and inversely 
to the square of the distance between them, enables Science, 
by a mathematical process, not only to determine the order of 
the motions of the heavenly bodies, but also the perturbations 
of that order, and by accurate observations to verify the con- 
clusions ; and it has enabled mathematicians not only to ex- 
plain phenomena already observed, but even to discover the 
existence, and determine the conditions, of others not yet 
observed. It is obvious that a general law of this kind has 
an authority which no merely phenomenal law can possess. 
Its discovery — or rather, I should say, its application — is a far 
higher act of human reason ; its accuracy may be tested to an 
almost unlimited extent by the aid of mathematics ; and we 
cannot but accept the law as a part of the established order 
of the universe which governs a very large class of secondary 
and phenomenal laws, and the determination of which is thus 
a long step in the direction of the interpretation of that order. 
But, observe, one step and nothing more. If gravitation is 
the cause of many effects in the order of the universe, what 
is the cause of gravitation ? We cannot be surprised that 
the natural feeling in the scientific mind is that some cause 
must exist in Nature itself. Newton himself considered that 
it was impossible for any one “ who has in philosophic matters 
a competent faculty of thinking,” to allow the possibility of 
action at a distance, such as gravity seems to imply. Yet 
none of the hypotheses, as yet suggested to account for 
gravity, except that of Le Sage, has any claim whatever to be 
a scientific exposition.* However, this only leaves us with a 
still more difficult question, viz., what can be the cause in 
Nature of ultra mundane corpuscles flying about in all possible 
directions, in infinite numbers, and with enormous velocity ? 
Sooner or later, it seems, we must get beyond Nature. A 
hypothesis of all this ultra mundane energy, of which only an 
infinitesimal part affects Nature at all, looks very like a 
confession of this truth. 
17. However, there is a more fundamental question still, to 
which I must briefly refer. It is well known that all the mathe- 
* Unseen Universe , Article, 140-141, 
