Since the days of Descartes it has been a conception familiar to philosophers that every visible event in nature might be explained by previous visib… - George Santayana

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Since the days of Descartes it has been a conception familiar to philosophers that every visible event in nature might be explained by previous visible events, and that all the motions, for instance, of the tongue in speech, or of the hand in painting, might have merely physical causes. If consciousness is thus accessory to life and not essential to it, the race of man might have existed upon the earth and acquired all the arts necessary for its subsistence without possessing a single sensation, idea, or emotion. Natural selection might have secured the survival of those automata which made useful reactions upon their environment. An instinct would have been developed, dangers would have been shunned without being feared, and injuries avenged without being felt.