It is equally impossible to discover when and under what precise circumstances the cornerstone of the theory was laid – the Law of Gravity, which states that the force of attraction is proportionate to the attracting masses, and diminishes with the square of the distance. It had been suggested, but without concrete proof, as far back as 1645 by Boulliau. Perhaps it was derived by analogy from the diffusion of light which, as Kepler knew, also diminishes in intensity with the square of distance. Another suggestion is that it was deduced from Kepler’s Third Law; Newton himself says that he found
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