Before 1914, most people still believed in Enlightenment ideals of progress, reason, and individual rights. Supporters of these philosophies had some cause for optimism. Women and workers were gradually gaining support in their struggles for political and social recognition, and the rising standard of living, the taming of the city, and the growth of state-supported social programs suggested that life was indeed improving.

Nevertheless, in the late nineteenth century, a small group of serious thinkers mounted a determined attack on these optimistic beliefs. The German philosopher Friedrich Nietzsche (NEE-chuh) (1844–1900) was particularly influential. In the first of his Untimely Meditations (1873), he argued that ever since classical Athens, the West had overemphasized rationality and stifled the authentic passions and animal instincts that drive human activity and true creativity.

Nietzsche believed that reason, progress, and respectability were outworn social and psychological constructs that suffocated self-realization and excellence. Though he was the son of a Lutheran minister, Nietzsche rejected religion. In his 1887 book, On the Genealogy of Morals, he claimed that Christianity embodied a “slave morality” that glorified weakness, envy, and mediocrity. In one of his most famous lines, an apparent madman proclaims that “God is dead,” metaphorically murdered by lackadaisical modern Christians who no longer really believed in him.

Nietzsche warned that Western society was entering a period of nihilism — the philosophical idea that human life is entirely without meaning, truth, or purpose. According to Nietzsche, the only hope for the individual was to accept the meaninglessness of human existence and then make that very meaninglessness a source of self-defined personal integrity and hence liberation. In this way, at least a few superior individuals could free themselves from the humdrum thinking of the masses and become true heroes.

Little read during his active years, Nietzsche’s works attracted growing attention in the early twentieth century. Artists and writers experimented with his ideas, which were fundamental to the rise of the philosophy of existentialism in the 1920s. Subsequent generations remade Nietzsche to suit their own needs, and his influence remains enormous to this day.

The growing dissatisfaction with established ideas before 1914 was apparent in other important thinkers as well. In the 1890s, French philosophy professor Henri Bergson (1859–1941), for one, argued that immediate experience and intuition were as important as rational and scientific thinking for understanding reality. According to Bergson, a religious experience or mystical poem was often more accessible to human comprehension than a scientific law or a mathematical equation.

The First World War accelerated the revolt against established certainties in philosophy, but that revolt went in two very different directions. In English-speaking countries, the main development was the acceptance of logical positivism in university circles. In the continental countries, the primary development in philosophy was existentialism.

Adherents of logical positivism argued that what we know about human life must be based on rational facts and direct observation. They concluded that theology and most traditional philosophy was meaningless because ideas about God, eternal truth, and ethics were impossible to prove using logic. This outlook is often associated with the Austrian philosopher Ludwig Wittgenstein (VIHT-guhn-shtine) (1889–1951), who later immigrated to England where he trained numerous disciples.

In his Tractatus Logico-Philosophicus (Essay on Logical Philosophy), published in 1922, Wittgenstein argued that philosophy is only the logical clarification of thoughts and that therefore it should concentrate on the study of language, which expresses thoughts. In his view, the great philosophical issues of the ages — God, freedom, morality, and so on — were quite literally senseless because neither science nor mathematics could demonstrate their validity. Logical positivism, which has remained dominant in England and the United States to this day, drastically reduced the scope of philosophical inquiry and offered little solace to ordinary people.

On the continent, others looked for answers in existentialism. This new philosophy loosely united highly diverse and even contradictory thinkers in a search for usable moral values in a world of anxiety and uncertainty. Modern existentialism had many nineteenth-century forerunners, including Nietzsche, the Danish religious philosopher Søren Kierkegaard (1813–1855), and the Russian novelist Fyodor Dostoyevsky (1821–1881). The philosophy gained recognition in Germany in the 1920s through the efforts of philosophers Martin Heidegger (1889–1976) and Karl Jaspers (1883–1969). These writers placed great emphasis on the loneliness and meaninglessness of human existence in a godless world and the individual’s need to come to terms with the fear caused by this situation.

Most existential thinkers in the twentieth century were atheists. They did not believe that a supreme being had established humanity’s fundamental nature and given life its meaning. In the words of French existentialist Jean-Paul Sartre (ZHAWN-pawl SAHR-truh) (1905–1980), “existence precedes essence.” By that, Sartre meant that there are no God-given, timeless truths outside or independent of individual existence. Only after they are born do people struggle to define their essence, entirely on their own. According to thinkers like Sartre and his life-long intellectual partner Simone de Beauvoir (1908–1986), human beings are terribly alone, for there is no God to help them. They are left to confront the inevitable arrival of death and so are hounded by despair.

At the same time, existentialists recognized that human beings must act in the world. Because life is meaningless, existentialists believe that individuals are forced to create their own meaning and define themselves through their actions. To live authentically, individuals must become “engaged” and choose their own actions in full awareness of their inescapable responsibility for their own behavior. Existentialism thus had a powerful ethical component. It placed great stress on individual responsibility and choice, on “being in the world” in the right way.