The Enlightenment was a political and philosophical movement that evolved in Europe in the seventeenth and eighteenth centuries, and crossed the Atlantic to play an important part in the creation of the United States. Empiricism had gained ground against the remains of medieval Christianity in the 1450-1650 period, and the successes of Isaac Newton and other scientists convinced an emerging generation of thinkers that the methods of science - reasoning about direct observation of nature rather than sacred texts - could be extended to all the questions people have about life and how to live it.
Replacing the Judeo-Christian god with Nature or a more or less abstract First Cause opened the way for secular humanism - the idea that man was free to exist for his own sake, and to frame all questions of ethics and values in terms of human life rather than obedience to god-given rules. The idea of the divine right of kings, which had dominated European politics for centuries, was no longer tolerable; the Enlightenment sought to promote the "Rights of Man." Different writers and political parties specified these rights in different ways, but the basic values underlying them were consistent. The Enlightenment was against arbitrary power, religious superstition, and the hierarchies they supported (as exemplified by the aristocracy and clergy of pre-Revolutionary France.) It was for free enquiry, the study of nature, individual autonomy, private property, equality under law, and, above all, liberty.
...are not necessarily universal.Edit
The radicalism of this new view of the relationship of man to society and nature is probably more obvious to religious conservatives than to modern secular humanists, who have settled into a consensus understanding of "human rights" as something like the US Constitution's "Bill of Rights" with the possible addition of some economic rights. But if we want to treat these rights as something definite and certain, we have to know how we can know what they are.
While the US Declaration of Independence simplifies matters with the assertion that it's "self-evident, that all Men are ... endowed, by their Creator, with certain unalienable Rights," this natural rights approach to ethics assumes the action of a supernatural being, the existence of which was questioned or rejected by important thinkers of the era. For example, David Hume and the Marquis de Sade were both atheists, and both concluded that there was nothing in nature to dictate an ethical system to humanity. Sade, seeing no rights in nature except for the right of the strong to rule the weak, saw nihilism as the logical consequence. Hume likewise found no logical basis for any particular ethical system, but argued that habit, convention, and experience always had been and would continue to be the true origin of ethical systems.
Another theory of the origin of rights is the idea of the "social contract" as presented by John Locke and Jean-Jacques Rousseau, among others. Advocates of this theory envision a human race beginning in the brutal every-man-for-himself mode of Sade's philosophy, and want no part of it. They claim that humanity, in pursuit of a better way of life, has agreed to an implicit "contract" through which people grant certain rights to their neighbors in order to get the same rights for themselves. This is a reasonable metaphor, but since no one really has the chance to sign such a contract, the details of the rights and duties it might define will remain in doubt.
Immanuel Kant differed from most Enlightenment philosophers by subordinating empiricism to his own version of Cartesian rationalism. He was influenced by Hume's skepticism, but was unwilling to accept either the uncertainty of Hume's worldview or the a priori assumptions of earlier rationalists. He attempted to restore certainty to the world by developing laws of thought that were independent of experience, yet integral to any process by which one could gain knowledge of the world. Although his approach to philosophy was immensely influential, there remains much disagreement about what he was trying to prove and how successfully he proved it.
If the Enlightenment had succeeded in reducing ethics to a science, we would have a detailed and indisputable description of the Rights of Man that we could apply to any situation, confident that we could always construct one correct solution to any problem we can describe. That didn't happen, and ethical systems remain somewhat arbitrarily chosen human constructions. Secular humanists will remain at odds with advocates of faith-based or otherwise inflexible ethical systems, despite their own usual statements of support for freedom of religion. It is this fact that makes it unreasonable to attempt to transplant "Enlightenment values" into obvious theocracies, tribal societies, or those parts of Asia where variants of Confucianism have historically worked to promote social harmony rather than individualism.