Kant’s philosophy dominated the thought of the 19th century, and Durant admits he towers above the 19th-century thinkers he will later cover. The transition from Voltaire to Kant is one “from theoretical reason without religious faith, to religious faith without theoretical reason” (277). The animating conviction of the Enlightenment was in the power of human reason to transform nature, both in terms of physical reality and human society. The first sign of dissent was the question of whether it was possible for reason to triumph over superstition and other irrational forces, especially once philosophers like John Locke and David Hume developed theories of psychology which highlighted the inherent limits of human perception.

Hume’s skepticism regarding the inability of human beings to detect the real laws of the universe threatened science no less than religion. Rousseau took this even further by arguing that reason-driven material progress had ruined mankind’s primordial state, where they lived simple and happy lives according to instinct. Rousseau argued that even education, which the Enlightenment had held up as the key to humanity’s liberation, “does not make a man good, it only makes him clever—usually for mischief” (284).