Charles-Louis de Secondat, Baron de Montesquieu, was a French political philosopher whose Spirit of the Laws (1748) profoundly influenced constitutional design, including the U.S. Constitution. A member of the French aristocracy and magistrate in Bordeaux, Montesquieu brought comparative and empirical methods to political philosophy.
The Spirit of the Laws analyzed governments across history and around the world, arguing that political systems should fit their society's climate, customs, and circumstances. His most influential concept was separation of powers—dividing government into legislative, executive, and judicial branches to prevent tyranny. His analysis of the English constitution (idealized, but influential) provided the model.
Montesquieu also developed a theory of political liberty as security under law, distinguishing it from mere absence of restraint. He opposed slavery, advocated religious tolerance, and criticized despotism. His combination of philosophical principle with sociological observation made him a founder of political science as well as constitutionalism.