Enlightenment
The 17th and 18th century movement that made reason, not kings or scripture, the judge of truth and government, and helped topple thrones.
When Thomas Jefferson wrote that governments derive 'their just powers from the consent of the governed,' he was not inventing an argument. He was summarizing a movement roughly a century old. The Enlightenment was the claim, radical in an age of divine-right kings, that human reason rather than tradition, scripture, or royal authority should be the final judge of what is true and how a society should be run. It ran, by convention, from the 1680s to the age of Napoleon, and it supplied the ideas that blew up thrones on two continents.
Reason against tradition
The movement grew out of the Scientific Revolution. If Isaac Newton could explain the motion of the planets with a handful of equations in his Principia (1687), thinkers reasoned, the same tool of systematic reason could be turned on law, religion, economics, and government. The result was a generation of writers, the French called them the philosophes, who treated no inherited authority as beyond question.
Their arguments still structure modern politics. John Locke held that people possess natural rights to life, liberty, and property, and that legitimate government rests on the consent of the governed, who may replace a ruler who breaks the bargain. Montesquieu argued for separating a government's powers so that ambition would check ambition. Rousseau located sovereignty in the people themselves and their collective general will. Voltaire hammered at religious persecution and censorship, making tolerance and free expression fashionable. Adam Smith argued that a nation grew richest when individuals were left free to truck, barter, and exchange.
From salon to revolution
These ideas spread through a new social infrastructure. Parisian salons hosted by women such as Madame Geoffrin, London coffeehouses, and cheap pamphlets carried the arguments past the universities and into polite society. The monument of the age was Denis Diderot's Encyclopedie, a 28-volume project begun in 1751 to gather all human knowledge in one place and, quietly, to smuggle skeptical and reformist ideas past the censors.
Then the abstractions became law. The American Declaration of Independence (1776) reads like Locke set to music, and the Constitution of 1787 built Montesquieu's separated powers into working machinery. In 1789 the French Declaration of the Rights of Man and of the Citizen proclaimed that men are born free and equal in rights. States were being founded not on dynasty or conquest but on written principles a philosopher could have drafted.
The double edge
That was the triumph; the shadow followed close behind. The same confidence that reason could redesign society from first principles also armed the radicals who guillotined thousands during the Terror of 1793 and 1794 in the name of virtue, and it echoed in the later utopian violence of regimes certain they had found the rational blueprint for humanity. The Enlightenment gave the modern world its best furniture, human rights, government by consent, and freedom of thought, and also its most dangerous temptation, the belief that clever people with a correct theory may remake everything at once.
Every time a court voids a law for violating individual rights, an argument first sharpened in an 18th-century salon is still doing its work.