The Age of Enlightenment (French: Siècle des Lumières; German: Aufklärung) was an eighteenth century movement in European and American philosophy, or the longer period including the Age of Reason. The term can more narrowly refer to the intellectual movement of The Enlightenment, which advocated Reason as the primary basis of authority. Developing in France, Britain and Germany, its sphere of influence also included Austria, Italy, the Netherlands, Poland, Russia, Scandinavia, Spain and, in fact, the whole of Europe. Many of the United States' Founding Fathers were also heavily influenced by Enlightenment-era ideas, particularly in the religious sphere (Deism) and, in parallel with classical liberalism, in the political sphere (which had a major influence on its Bill of Rights, in parallel with the Declaration of the Rights of Man and the Citizen).
The era is generally agreed to have ended around the year 1800 and the beginning of the Napoleonic Wars (1804-15).

History
As with theology, philosophy became a source of partisan debate, with different schools attempting to develop rationales for their viewpoints. Philosophers such as Spinoza searched for a metaphysics of ethics, which influenced pietism and the transcendental philosophy of philosophers such as Immanuel Kant.
Religion was linked to another concept which inspired a great amount of Enlightenment thought, namely the rise of the Nation-state. In the medieval and Renaissance periods, the state was restricted by the need to work through a host of intermediaries. This system existed because of poor communication, where localism thrived in return for loyalty to some central organization. Following improvements in transportation, organization, navigation and finally the influx of gold and silver from trade and conquest, however, the state assumed more authority and power. Intellectuals responded with a series of theories on the purpose and limit of state power. Throughout The Enlightenment, absolutism was therefore cemented. A string of philosophers (amongst them John Locke) reacted by advocating limitations on legitimate state power, influencing both Voltaire and Jean-Jacques Rousseau. The influence of these Enlightenment ideas extended to organizations seeking to affect state and social development and ultimately had a profound effect on the actions of politically active individuals worldwide.
Within the period of the Enlightenment, the question of what was the proper relationship of the citizen to the state continued to be explored. The idea that society is a contract between individual and some larger entity, whether society or state, was developed philosophically by a series of thinkers, including Rousseau, Montesquieu and Jefferson. Other thinkers, heralding romanticism, advocated the idea that nationality had a basis beyond mere preference. Philosophers such as Johann Gottfried von Herder expounded the idea that language had a decisive influence on cognition and thought, and that the meaning of a particular book or text was open to deeper exploration based on deeper connections, an idea now called hermeneutics. The two concepts -- of the contractual nature between the state and the citizen, and the reality of the nation beyond that contract -- had a decisive influence in the development of liberalism, democracy and constitutional government which followed.
At the same time, the integration of algebraic thinking, acquired from the Islamic world over the previous two centuries, and geometric thinking which had dominated Western mathematics and philosophy since at least Eudoxus, precipitated a scientific and mathematical revolution. Sir Isaac Newton's greatest claim to prominence came from a systematic application of algebra to geometry, and synthesizing a workable calculus which was applicable to scientific problems. The Enlightenment was a time when the solar system was truly discovered: with the accurate calculation of orbits, such as Halley's comet, the discovery of the first planet since antiquity, Uranus by William Herschel, and the calculation of the mass of the Sun using Newton's theory of universal gravitation. These series of discoveries had a momentous effect on both pragmatic commerce and philosophy. The excitement engendered by creating a new and orderly vision of the world, as well as the need for a philosophy of science which could encompass the new discoveries, greatly influenced both religious and secular ideas. If Newton could order the cosmos with natural philosophy, so, many argued, could political philosophy order the body politic.
Within the Enlightenment, two main theories contended to be the basis of that ordering: divine right and natural law. The writings of Jacques-Benigne Bossuet (1627-1704) set the paradigm for the divine right: that the universe was ordered by a reasonable God, and therefore his representative on earth had the powers of that God. The orderliness of the cosmos was seen as proof of God; therefore it was a proof of the power of monarchy. Natural law began, not as a reaction against divinity, but instead, as an abstraction: God did not rule arbitrarily, but through natural laws that he enacted on earth. Thomas Hobbes, though an absolutist in government, drew on this argument in Leviathan. Once the concept of natural law was invoked, however, it took on a life of its own. If natural law could be used to bolster the position of the monarchy, it could also be used to assert the rights of subjects of that monarch. If there were natural laws, then there were natural rights associated with them, just as there are rights under man-made laws.
What both theories had in common was the need for an orderly and comprehensible function of government. The "Enlightened Despotism" of, for example, Catherine the Great of Russia and Frederick the Great of Prussia, was not based on mystical appeals to authority, but on the pragmatic invocation of state power as necessary in order to hold back the anarchy of warfare and rebellion. Regularization and standardization were seen as good things because they allowed the state to reach its power outwards over the entirety of its domain and because they liberated people from being entangled in endless local custom. Additionally, they expanded the sphere of economic and social activity.
Thus rationalization, standardization and the search for fundamental unities occupied much of the Enlightenment and its arguments over proper methodology and nature of understanding. The culminating efforts of the Enlightenment include, amongst other things, the economics of Adam Smith, the physical chemistry of Antoine Lavoisier, the idea of evolution pursued by Johann Wolfgang Goethe and the declaration by Jefferson of inalienable rights. Development in the philosophy of the Enlightenment was also the basis for overthrowing the idea of a completely rational and comprehensible universe, and led, in turn, to the metaphysics of Hegel and Romanticism.

Conflicts
The Enlightenment occupies a central role in the justification for the movement known as modernism. The neo-classicizing trend in modernism came to see itself as a period of rationality which overturned established traditions, analogously to the Encyclopaediasts and other Enlightenment philosophers. A variety of 20th century movements, including liberalism and neo-classicism, traced their intellectual heritage back to the Enlightenment, and away from the purported emotionalism of the 19th century. Geometric order, rigor and reductionism were seen as Enlightenment virtues. The modern movement points to reductionism and rationality as crucial aspects of Enlightenment thinking, of which it is the heir, as opposed to irrationality and emotionalism. In this view, the Enlightenment represents the basis for modern ideas of liberalism against superstition and intolerance. Influential philosophers who have held this view include Jürgen Habermas and Isaiah Berlin.
This view asserts that the Enlightenment was the point when Europe broke through what historian Peter Gay calls "the sacred circle," whose dogma had circumscribed thinking. The Enlightenment is held to be the source of critical ideas, such as the centrality of freedom, democracy and reason as primary values of society. This view argues that the establishment of a contractual basis of rights would lead to the market mechanism and capitalism, the scientific method, religious tolerance, and the organization of states into self-governing republics through democratic means. In this view, the tendency of the philosophes in particular to apply rationality to every problem is considered the essential change. From this point on, thinkers and writers were held to be free to pursue the truth in whatever form, without the threat of sanction for violating established ideas.
With the end of the Second World War and the rise of post-modernity, these same features came to be regarded as liabilities - excessive specialization, failure to heed traditional wisdom or provide for unintended consequences, and the romanticization of Enlightenment figures - such as the Founding Fathers of the United States, prompted a backlash against both Science and Enlightenment based dogma in general. Philosophers such as Michel Foucault are often understood as arguing that the Age of Reason had to construct a vision of unreason as being demonic and subhuman, and therefore evil and befouling, whence by analogy to argue that rationalism in the modern period is, likewise, a construction. In their book, Dialectic of Enlightenment, Max Horkheimer and Theodor Adorno wrote a critique of what they perceived as the contradictions of Enlightenment thought: Enlightenment was seen as being at once liberatory and through the domination of instrumental rationality, tending towards totalitarianism.
Still yet, other leading intellectuals, such as Noam Chomsky, see a natural evolution, using the term loosely, from early Enlightenment thinking to other forms of social analysis, specifically from The Enlightenment to liberalism, anarchism and socialism. The relationship between these different schools of thought, Chomsky and others point out , can be seen in the works of von Humboldt, Kropotkin, Bakunin and Marx, among others.

See also

Henry F. May The Enlightenment in America (New York: Oxford University Press, 1976)
Ernst Cassirer, The Philosophy of the Enlightenment, Princeton University Press 1979
Mark Hulluing Autocritique of Enlightenment: Rousseau and the Philosophes 1994
Gay Peter. The Enlightenment: An Interpretation. New York: W. W. Norton & Company, 1996
Michel Foucault, What is enlightenment?
Redkop, Benjamin, The Enlightenment and Community, 1999
Melamed, Yitzhak Y, Salomon Maimon and the Rise of Spinozism in German Idealism, Journal of the History of Philosophy, Volume 42, Issue 1
Porter, Roy The Enlightenment 1999
Jacob, Margaret Enlightenment: A Brief History with Documents 2000
Thomas Munck Enlightenment: A Comparative Social History, 1721-1794
Arthur Herman How the Scots Invented the Modern World: The True Story of how Western Europe's Poorest Nation Created Our World and Everything in It 2001
Stuart Brown ed., British Philosophy in the Age of Enlightenment 2002
Alan Charles Kors, ed. Encyclopedia of the Enlightenment. 4 volumes. Oxford: Oxford University Press, 2003
Buchan, James Crowded with Genius: The Scottish Enlightenment: Edinburgh's Moment of the Mind 2003
Bernard Dieterle, Manfred Engel (ed.), The Dream and the Enlightenment / Le Rêve et les Lumières. Paris: Honoré Champion 2003, ISBN 2-7453-0672-3.
Louis Dupre The Enlightenment & the Intellctural Foundations of Modern Culture 2004
Himmelfarb, Gertrude The Roads to Modernity: The British, French, and American Enlightenments, 2004
Stephen Eric Bronner Interpreting the Enlightenment: Metaphysics, Critique, and Politics, 2004
Jonathan Hill, Faith in the Age of Reason, Lion/Intervarsity Press 2004
Stephen Eric Bronner The Great Divide: The Enlightenment and its Critics
The London Philosophy Study Guide offers many suggestions on what to read, depending on the student's familiarity with the subject: Modern Philosophy