"Of habit, the power that keeps the earth from flying to pieces; though there is some silly theory of gravitation."

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The test of science is not whether you are reasonable—there would not be much of physics if that was the case—the test is whether it works. And the great point about Newton’s theory of gravitation was that it worked, that you could actually say something about the motion of the moon without knowing very much about the constitution of the Earth.

— Hermann Bondi