Be this as it may, there now arose another man greater than his predecessors, and greater than all his contemporaries; he also was an Englishman, by name Isaac Newton. What others guessed, or concluded on insufficient evidence, became, in his powerful hands, clear and well-grounded truths, proved, so far as such things could be proved, by rigid mathematical reasoning, and established on a solid basis, which time has not shaken, and which subsequent investigation has confirmed. Others had supposed the existence of the law of attraction by which the Sun acted on the planets; many persons had understood the existence of terrestrial gravitation. Newton showed that these two are identical; and, moreover, that every particle of matter attracts every other particle _mutually_, and according to the one universal law, that of the inverse square of the distance; so that a vast planet revolving round the Sun obeys the same law as a pebble dropped from one’s hand to the Earth. The popular story of his having been suddenly led to this conclusion by the sight of an apple falling is apparently fabulous; and what really occurred is this: he sat alone one day in a garden, and fell into a speculation (as men of scientific mind are apt to do) on the power of gravity, that is, of gravity as we feel it here on the Earth. Then it struck him that however high you ascend, even on the loftiest mountains, no sensible diminution in this remarkable force takes place; so, he said to himself: why not as high as the Moon? If so, perhaps she is retained in her orbit by this very power. And again if so, what then? To which question his active mind gave the just and true answer, that it was probably one and the same force that acted at the surface of the Earth, at the distance of the Moon, and finally, as regulating the action of the Sun on the planets.
It seems that there was an error, which it is unnecessary to explain in detail, in Newton’s first calculations; but that when, after a lapse of time and with the error corrected, he again returned to them, he found the motion of the Moon to be exactly accounted for by his theory.
Again, in dealing with the complicated problem of the action of the heavenly bodies one upon the other, that is, when the disturbing force, for instance, of a third body is brought to bear on the motions of two others, although Hooke and others had as a conjecture put forth the existence of such mutual action, yet Newton was the first who thoroughly grappled with it.