A Puzzle in Aristotle: Why Must There Be an Unmoved Mover?

A well-known doctrine of Aristotle’s is that of the unmoved mover, the first cause of the eternal motion of the universe. Aristotle’s key argument for the necessity of an unmoved mover is presented in Physics, Book VIII, chapters 4–5, where he argues, first, that everything that is in motion must be moved by something (ch. 4) and, second, that a thing cannot move itself (ch. 5). I have long been puzzled by this argument, and lately I have come to think it’s not just my ignorance—there really is something wrong with the argument. My aim here is to explain what it is.

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