If a man sees a river run downhill day after day and year after year, he is justified in reckoning, we might say in betting, that it will do so till he dies. But he is not justified in saying that it cannot run uphill, until he really knows why it runs downhill. To say it does so by gravitation answers the physical but not the philosophical question. It only repeats that there is a repetition; it does not touch the deeper question of whether that repetition could be altered by anything outside it. And that depends on whether there is anything outside it. For instance, suppose that a man had
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