The perspectives of the Enlightenment were challenged not only by romanticism and religious “enthusiasm” but also by the continued development of European science itself. This remarkable phenomenon justifies a brief look ahead at several scientific developments in the nineteenth and twentieth centuries.

Modern science was a cumulative and self-critical enterprise, which in the nineteenth century and later was applied to new domains of human inquiry in ways that undermined some of the assumptions of the Enlightenment. In the realm of biology, for example, Charles Darwin (1809–1882) laid out a complex argument that all life was in constant change, that an endless and competitive struggle for survival over millions of years constantly generated new species of plants and animals, while casting others into extinction. Human beings were not excluded from this vast process, for they too were the work of evolution operating through natural selection. Darwin’s famous books The Origin of Species (1859) and The Descent of Man (1871) were threatening to many traditional Christian believers, perhaps more so than Copernicus’s ideas about a sun-centered universe had been several centuries earlier.

At the same time, Karl Marx (1818–1883) articulated a view of human history that likewise emphasized change and struggle. Conflicting social classes—slave owners and slaves, nobles and peasants, capitalists and workers—successively drove the process of historical transformation. Although he was describing the evolution of human civilization, Marx saw himself as a scientist. He based his theories on extensive historical research; like Newton and Darwin, he sought to formulate general laws that would explain events in a rational way. Nor did he believe in heavenly intervention, chance, or the divinely endowed powers of kings. The coming of socialism, in this view, was not simply a good idea; it was inscribed in the laws of historical development. (See Working with Evidence, Source 17.1.)

Like the intellectuals of the Enlightenment, Darwin and Marx believed strongly in progress, but in their thinking, conflict and struggle rather than reason and education were the motors of progress. The Enlightenment image of the thoughtful, rational, and independent individual was fading. Individuals—plant, animal, and human alike—were now viewed as enmeshed in vast systems of biological, economic, and social conflict.

The work of the Viennese doctor Sigmund Freud (1856–1939) applied scientific techniques to the operation of the human mind and emotions and in doing so cast further doubt on Enlightenment conceptions of human rationality. At the core of each person, Freud argued, lay primal impulses toward sexuality and aggression, which were only barely held in check by the thin veneer of social conscience derived from civilization. Our neuroses arose from the ceaseless struggle between our irrational drives and the claims of conscience. This too was a far cry from the Enlightenment conception of the human condition.

And in the twentieth century, developments in physics, such as relativity and quantum theory, called into question some of the established verities of the Newtonian view of the world, particularly at the subatomic level and at speeds approaching that of light. In this new physics, time is relative to the position of the observer; space can warp and light can bend; matter and energy are equivalent; black holes and dark matter abound; and probability, not certain prediction, is the best that scientists can hope for. None of this was even on the horizon of those who made the original Scientific Revolution in the early modern era.