Jean-Jacques Rousseau was what might now be called a contrarian: His belief (as argued in his First and Second Discourses) was that the then-rapid advances in the sciences and arts were not a sign of the limitless possibilities opening up before the human race during the Enlightenment but rather an indication of a further fall from grace dramatically differed from the thinking of most of his contemporaries such as Voltaire and Diderot. This was true despite the fact that Rousseau in other ways very much a child and a philosopher of the Enlightenment, being most intensely interested with rights - and liberties - of individual citizens.
Much of Rousseau's writing in his "Discourses" (as elsewhere) centers on the core ideas of Naturalism, a rather unpopular philosophy (then and now) that argues that science, art, and social institutions have corrupted all of us and that the natural, or primitive or original state of humanity (that state we enjoy at birth) is morally superior to the civilized state.
It is not entirely clear to what extent Rousseau was at least initially committed to this idea: He began to write on this theme initially not out of any obviously well-developed commitment to reversing the Enlightenment emphasis on the uplifting virtues of scientific inquiry and artistic creation but rather because of a competition sponsored by the Dijon Academy on the subject of whether science and art in 18th-century France were contributing to the "purification or the corruption" of the morals of the people. He won the prize for his "Discours sur les sciences et les arts" and this established him as the First Romantic of the Age of Enlightenment, a man looking backward and inward even as everyone else was looking forward and outward. He argued for a philosophical perspective that would dominate much of both art and literature during the 19th century, the Romantic idea that we as humans are inherently good and that we have each been co...