Saying we are all really pursuing our own self-interest provides a way to cut past the welter of passions and emotions that seem to govern our daily existence, and to motivate most of what we actually observe people to do (not only out of love and amity, but also envy, spite, devotion, pity, lust, embarrassment, torpor, indignation, and pride) and discover that, despite all this, most really important decisions are based on the rational calculation of material advantage—which means that they are fairly predictable as well. “Just as the physical world is ruled by the laws of movement,” wrote
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