 What makes for the unity of such tremendously diverse thinkers under the label of “Enlightenment”? For the purposes of this entry, the Enlightenment is conceived broadly. D’Alembert, a leading figure of the French Enlightenment, characterizes his eighteenth century, in the midst of it, as “the century of philosophy par excellence”, because of the tremendous intellectual and scientific progress of the age, but also because of the expectation of the age that philosophy (in the broad sense of the time, which includes the natural and social sciences) would dramatically improve human life. Guided by D’Alembert’s characterization of his century, the Enlightenment is conceived here as having its primary origin in the scientific revolution of the 16th and 17th centuries. The rise of the new science progressively undermines not only the ancient geocentric conception of the cosmos, but also the set of presuppositions that had served to constrain and guide philosophical inquiry in the earlier times. The dramatic success of the new science in explaining the natural world promotes philosophy from a handmaiden of theology, constrained by its purposes and methods, to an independent force with the power and authority to challenge the old and construct the new, in the realms both of theory and practice, on the basis of its own principles. Taking as the core of the Enlightenment the aspiration for intellectual progress, and the belief in the power of such progress to improve human society and individual lives, this entry includes descriptions of relevant aspects of the thought of earlier thinkers, such as Hobbes, Locke, Descartes, Bayle, Leibniz, and Spinoza, thinkers whose contributions are indispensable to understanding the eighteenth century as “the century of philosophy par excellence”.