Consider this simple experiment with a laboratory rat: if you put both food and an electrical shock at the end of an alley, the rat finds himself stuck at a certain distance from the end. He begins to approach but withdraws; he begins to withdraw but finds the courage to approach again. He oscillates, conflicted.8 If you outfit the rat with a little harness to measure the force with which he pulls toward food alone and, separately, you measure the force with which he pulls away from an electric shock alone, you find that the rat gets stuck at the point where the two forces are equal and cancel
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