The Enlightenment included a range of ideas centered on reason as the primary source of authority and legitimacy and came to advance ideals like liberty, progress, tolerance, fraternity, constitutional government and separation of church and state. In France, the central doctrines of the Enlightenment philosophers were individual liberty and religious tolerance, in opposition to an absolute monarchy and the fixed dogmas of the Roman Catholic Church. The Enlightenment was marked by an emphasis on the scientific method and reductionism, along with increased questioning of religious orthodoxy—an attitude captured by the phrase Sapere aude, "Dare to know".
French historians traditionally place the Enlightenment between 1715 (the year that Louis XIV died) and 1789 (the beginning of the French Revolution). Some recent historians begin the period in the 1620s, with the start of the scientific revolution. Les philosophes (French for "the philosophers") of the period widely circulated their ideas through meetings at scientific academies, Masonic lodges, literary salons, coffee houses and in printed books and pamphlets. The ideas of the Enlightenment undermined the authority of the monarchy and the Church and paved the way for the political revolutions of the 18th and 19th centuries. A variety of 19th-century movements, including liberalism and neo-classicism, trace their intellectual heritage to the Enlightenment.
The Age of Enlightenment was preceded by and closely associated with the scientific revolution. Earlier philosophers whose work influenced the Enlightenment included Bacon, Descartes, Locke, and Spinoza. The major figures of the Enlightenment included Beccaria, Diderot, Hume, Kant, Montesquieu, Rousseau, Adam Smith, and Voltaire. Some European rulers, including Catherine II of Russia, Joseph II of Austria and Frederick II of Prussia, tried to apply Enlightenment thought on religious and political tolerance, which became known as enlightened absolutism. Benjamin Franklin visited Europe repeatedly and contributed actively to the scientific and political debates there and brought the newest ideas back to Philadelphia. Thomas Jefferson closely followed European ideas and later incorporated some of the ideals of the Enlightenment into the Declaration of Independence (1776). One of his peers, James Madison, incorporated these ideals into the United States Constitution during its framing in 1787.
The most influential publication of the Enlightenment was the Encyclopédie (Encyclopaedia). Published between 1751 and 1772 in thirty-five volumes, it was compiled by Diderot, d'Alembert (until 1759) and a team of 150 scientists and philosophers. It helped spread the ideas of the Enlightenment across Europe and beyond. Other landmark publications were Voltaire's Dictionnaire philosophique (Philosophical Dictionary; 1764) and Letters on the English (1733); Rousseau's Discourse on Inequality (1754) and The Social Contract (1762); Adam Smith's The Wealth of Nations (1776); and Montesquieu's The Spirit of the Laws (1748). The ideas of the Enlightenment played a major role in inspiring the French Revolution, which began in 1789. After the Revolution, the Enlightenment was followed by the intellectual movement known as Romanticism.
René Descartes' rationalist philosophy laid the foundation for enlightenment thinking. His attempt to construct the sciences on a secure metaphysical foundation was not as successful as his method of doubt applied in philosophic areas leading to a dualistic doctrine of mind and matter. His skepticism was refined by John Locke's Essay Concerning Human Understanding (1690) and David Hume's writings in the 1740s. His dualism was challenged by Spinoza's uncompromising assertion of the unity of matter in his Tractatus (1670) and Ethics (1677).
These laid down two distinct lines of Enlightenment thought: first, the moderate variety, following Descartes, Locke and Christian Wolff, which sought accommodation between reform and the traditional systems of power and faith, and second, the radical enlightenment, inspired by the philosophy of Spinoza, advocating democracy, individual liberty, freedom of expression and eradication of religious authority. The moderate variety tended to be deistic, whereas the radical tendency separated the basis of morality entirely from theology. Both lines of thought were eventually opposed by a conservative Counter-Enlightenment, which sought a return to faith.
In the mid-18th century, Paris became the center of an explosion of philosophic and scientific activity challenging traditional doctrines and dogmas. The philosophic movement was led by Voltaire and Jean-Jacques Rousseau, who argued for a society based upon reason rather than faith and Catholic doctrine, for a new civil order based on natural law, and for science based on experiments and observation. The political philosopher Montesquieu introduced the idea of a separation of powers in a government, a concept which was enthusiastically adopted by the authors of the United States Constitution. While the Philosophes of the French Enlightenment were not revolutionaries and many were members of the nobility, their ideas played an important part in undermining the legitimacy of the Old Regime and shaping the French Revolution.