Montesquieu, Charles Louis de Secondat, Baron de la Brède et de (1689–1755), a French political philosopher. His De l'Esprit des Lois (The Spirit of the Laws), published in 1748, is one of the greatest political treatises ever written. Montesquieu compared various types of government, principally what he called “republic,” “monarchy,” and “despotism.” He concluded that the freedom of the individual could best be guaranteed under a government in which the three branches—legislative, executive, and judicial—were separated so that the powers of each could check and balance one another. Montesquieu's theory of separation of powers was profoundly influential, becoming one of the principles of the United States Constitution and many later constitutions.

Montesquieu was born into an aristocratic family. He inherited his title from an uncle. After being educated in the law, he was a counselor in the Bordeaux parlement (law court), 1714–16, and the president of that tribunal, 1716–26. In 1721 Montesquieu published his Persian Letters, in which he made fun of French customs and civilization, as supposedly seen through the eyes of two Persian travelers. In 1728 he was elected to the French Academy.

From 1728 to 1731 Montesquieu traveled on the continent and spent some months in England. He became an admirer of the English government because of the freedoms it provided the English people. Montesquieu spent most of his later years in study and writing. His Considerations on the Causes of the Grandeur and of the Decadence of the Romans (1734) was an important contribution to the philosophy of history.