Enlightenment thought reflected the influences of the scientific revolution (particularly Newton) as well as Renaissance humanism. Both of the latter can in turn be attributed, at least in part, to the growth of Aristotelian thought toward the end of the Middle Ages, which encouraged thinkers to renounce their previous obsession with the otherworldly and to focus instead upon things in the real world, including human beings. Enlightenment philosophers believed strongly in the existence of natural laws, amenable to human reason and observation. They did not hesitate to question traditional, faith-based beliefs, including in particular the teachings of the Church. The spirit of this movement was fundamentally secular, in contrast to the religious views dominant in medieval times. Furthermore, these philosophers believed that their discovery of the power of reason represented a major stride forward, which would lead to great progress in the lives of human beings; for this reason, this intellectual period has been dubbed the Age of Enlightenment by later historians.

The foremost European political theorist in this stage of liberalism was John Locke (1632-1704), who set forth his political views in his Two Treatises of Civil Government (1690). Locke's epistemological premises were roughly comparable to the views presented in Section 1 of this course (sans Rand's recent insights into the theory of concepts, of course). Knowledge, he contended in his Essay Concerning Human Understanding (1690), comes from experience combined with reflection, rather than from innate ideas or divine revelation.