“So,” I would say, parroting their answers back to them, “things fall because of gravity, but gravity is just the tendency for things to fall. Isn’t that circular and vacuous? What produces that tendency to fall? If we don’t know, have we really identified the cause of gravity? Or have we just treated the name of the effect in question as its own cause?” The puzzled looks on the faces of my students confirmed that they had begun to understand exactly why gravitational action at a distance bothered Leibniz. Newton only exacerbated Leibniz’s concern by acknowledging explicitly that he “feigned”
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