On The Spirit of the Laws (1748) by Charles Louis de Secondat, aka Baron de Montesquieu.
What keeps a society functioning? Montesquieu, though of course not the first political philosopher, was perhaps the first to systematically explore correlations between characteristics of a government, its people, its climate, dominant industries, religion, and other factors. Some of his ideas directly influenced the American Constitution, and some of them are very very weird.
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