Charles-Louis de Secondat, Baron de La Brède et de Montesquieu — French philosophe, lawyer, and comparative political theorist whose The Spirit of the Laws (1748) is one of the most influential single works of political analysis ever written. Its approach was novel: rather than deducing the best regime from first principles, Montesquieu surveyed actual governments — despotic, monarchical, republican — and asked how each was shaped by climate, geography, custom, religion, commerce, and the temperament of its people. Political forms, he argued, are not interchangeable; they fit their conditions or they fail.
Two arguments from the book shaped the century that followed. The first is his theory of the separation of powers: liberty depends on dividing legislative, executive, and judicial functions among distinct bodies so that power checks power. His (somewhat idealized) reading of the English constitution as the exemplar carried this argument across the Atlantic, where it shaped the U.S. Constitution — Madison's Federalist 47 cites Montesquieu as "the oracle" on the subject. The second is his defense of commerce as a civilizing force ("doux commerce"): trade softens manners, makes war costly, and creates interdependencies that restrain arbitrary power. Later critics would question the idealization, but the thought became a staple of liberal internationalism.
Alexis de Tocqueville inherited Montesquieu's method — empirical, comparative, attentive to the social undergrowth of political forms — and extended it to democratic America a century later.