I have tried to show that without his invasion into the territory of physics Kepler could not have succeeded. I must now discuss briefly Kepler’s particular brand of physics. It was, as to be expected, physics-on-the-watershed, half-way between Aristotle and Newton. The essential concept of impetus or momentum, which makes a moving body persist in its motion without the help of an external force, is absent from it; the planets must still be dragged through the ether like a Greek oxcart through the mud. In this respect Kepler had not advanced further than Copernicus, and both were unaware of
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