Although chronologically Jean-Jacques Rousseau is considered an Enlightenment author, as stated in the previous response, he is often described as a counter-Enlightenment thinker, representing the Romantic revolt against the scientific and rational leanings of Enlightenment philosophy. In particular, in his Discourse on the Sciences and Arts, Rousseau makes the point that science did not improve morality nor contribute a great deal to happiness. He argues historically that the ages in which science has triumphed have not been ones distinguished by civic virtue or morality.
Unlike such philosophers as Locke and Hobbes, Rousseau does not see the growth of civilization as progress from a natural life that is "nasty, brutish, and short" but paints an almost Edenic (albeit secular) portrait of human nature in its original natural state. Much of his approach to civic and individual virtue, rather than emphasizing the pure reason valorized by Enlightenment ideals also argues for the importance of empathy as the grounds of morality.
The Enlightenment was a period of time when western thought and culture experienced massive revolutions. This intellectual movement included changes in science, economics, and politics, with an increased focus on rationalism. In fact, this period of time is often referred to as the Age of Reason. In his essay called "First Discourse", Rousseau critisized the Enlightenment, saying that as the arts and sciences advanced, manners and morals became corrupt. Rousseau argued that logic and reasoning had trumped feeling and religion. Many say that Rousseau was the start of the counter-Enlightenment, which attacked things like rationalism. He also inspired the Romantic movement, which called for a return to man's natural state. The Romantic movement emphasized imagination and passion.