Why is the ninth month called September?
1660s, "action of enlightening," from enlighten + -ment. Used only in figurative sense, of spiritual enlightenment, etc. Attested from 1865 as a translation of German Aufklärung, a name for the spirit and system of Continental philosophers in the 18c.
The philosophy of the Enlightenment insisted on man's essential autonomy: man is responsible to himself, to his own rational interests, to his self-development, and, by an inescapable extension, to the welfare of his fellow man. For the philosophes, man was not a sinner, at least not by nature; human nature -- and this argument was subversive, in fact revolutionary, in their day -- is by origin good, or at least neutral. Despite the undeniable power of man's antisocial passions, therefore, the individual may hope for improvement through his own efforts -- through education, participation in politics, activity in behalf of reform, but not through prayer. [Peter Gay, "The Enlightenment"]
An intellectual movement of the seventeenth and eighteenth centuries marked by a celebration of the powers of human reason, a keen interest in science, the promotion of religious toleration, and a desire to construct governments free of tyranny. Some of the major figures of the Enlightenment were David Hume, Immanuel Kant, John Locke, the Baron de Montesquieu, Jean-Jacques Rousseau, and Voltaire.