The Enlightenment was an 18th century period where people began using reason and rationality to understand the natural world, questioning traditional institutions. Key documents like the U.S. Bill of Rights and French Declaration of the Rights of Man advocated for liberty and checks on government power. Enlightenment thinkers like Immanuel Kant and Marquis de Montesquieu believed humans were designed to act rationally and opposed absolute monarchy. The movement spread new ideas through novels and works like Alexander Pope's An Essay on Man.