Montesquieu (1689–1755) was born at the Château de La Brède near Bordeaux, trained in law, and presided over the Parliament of Bordeaux. After resigning the post he travelled Europe for years, spending two of them in England closely observing the parliamentary system. In 1748, as the fruit of two decades' work, he published De l'esprit des lois in Geneva; within a year it ran to twenty-two editions, was placed on the Index by the Vatican, and set the political agenda of the Enlightenment.

The book's central thesis is simple and devastating. Whichever the regime — republic, monarchy, or despotism — liberty requires that the powers of making, executing, and judging the law not be held by the same hand or body. 'When the legislative and executive powers are united in the same person, or in the same body of magistrates, there can be no liberty; because apprehensions may arise that the same monarch or senate should enact tyrannical laws and execute them tyrannically.' This is an empirical claim drawn from observation of the English constitution, not an abstract doctrine.

Montesquieu also seeks causal links between climate, geography, social structure, and political regime — a move criticised today, but one that marks the birth of comparative political sociology. The 1787 United States Constitution, as James Madison explains in the Federalist Papers, rests directly on Montesquieu; the French Constitution of 1791 and every later liberal constitution inherits his frame. The phrase 'separation of powers' is his sentence; it is the structural principle of modern democracy.