Plato and Aristotle already drew attention to the capacity of animate beings to move themselves. But the Epicureans may have been the first to worry about the conditions under which this was possible. In particular, they worried that if everything in the universe happens as a matter of necessity, the world unfolding inevitably from the past to the present, then nothing would have a power of free volition. And they were right to worry. After all, their physics describes the world as the result of atomic collisions, each of which seems to be like one rock hitting another. This suggests that the
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