The second property of our intelligence is, that we can generalise many facts into one.  Tracing agreement among the multifarious appearances of things, we can comprehend in one statement a vast number of details.  The single law of gravity expresses the fall of a stone, the flow of rivers, the retention of the moon in her circuit round the earth.  Now, this generalising sweep is a real advance in our knowledge, an ascent in the matter of intelligence, a step towards centralising the empire of science.  What is more, this is the only real meaning of EXPLANATION.  A difficulty is solved, a mystery unriddled; when it can be shown to resemble something else; to be an example of a fact already known.  Mystery is isolation, exception, or, it may be, apparent contradiction; the resolution of the mystery is found in assimilation, identity, fraternity.  When all things are assimilated, so far as assimilation can go, so far as likeness holds, there is an end to explanation; there is an end to what the mind can do, or can intelligently desire.

[GRAVITY NOT A MYSTERY.]

Thus, when Gravity was generalised, by assimilating the terrestrial attraction seen in falling bodies with the celestial attraction of the sun and planets; and when, by fair presumption, the same power was extended to the remote stars; when, also, the law was ascertained, so that the movements of the various bodies could be computed and predicted, there was nothing further to be done; explanation was exhausted.  Unless we can find some other force to fraternise with gravity, so that the two might become a still more comprehensive unity, we must rest in gravity as the ultimatum of our faculties.  There is no conceivable modification, or substitute, that would better our position.  Before Newton, it was a mystery what kept the moon and the planets in their places; the assimilation with falling bodies was the solution.  But, say many persons, is not gravity itself a mystery?  We say No; gravity has passed through all the stages of legitimate and possible explanation; it is the most highly generalised of all physical facts, and by no assignable transformation could it be made more intelligible than it is.  It is singularly easy of comprehension; its law is exactly known; and, excepting the details of calculation, in its more complex workings, there is nothing to complain of, nothing to rectify, nothing to pretend ignorance about; it is the very pattern, the model, the consummation of knowledge.  The path of science, as exhibited in modern times, is towards generality, wider and wider, until we reach the highest, the widest laws of every department of things; there explanation is finished, mystery ends, perfect vision is gained.

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