Existentialism is not a single school of thought but rather a label applied to several systems that are influenced by the theories of Danish philosopher Soren Kierkegaard (1813–1855). Existentialist thinkers consider one problem: human existence in an unfathomable universe. However, in considering this “plight,” philosophers have arrived at different conclusions.

The founder of existentialism, Kierkegaard rejected the principles put forth by traditional philosophers such as Georg Hegel (1770–1831), who had considered philosophy as a science, asserting that it is both objective and certain. Kierkegaard overturned this assertion, citing that truth is not objective but rather subjective; that there is no such thing as universal truths; and that human existence is not understandable in scientific terms. He maintained that human beings must make their own choices, based on their own knowledge. When he wrote on the subject, Kierkegaard frequently used pseudonyms, a practice he defended by intimating that he was putting the onus on his readers to determine what is true—that they shouldn’t rely on the “authority” of his philosophies.

In the twentieth century, heirs to Kierkegaard’s school of thought included German philosopher Martin Heidegger (1889–1976), who rejected the label “existentialist,” and the French writer Jean-Paul Sartre (1905–1980), the only self-proclaimed existentialist. They grappled with the dilemma that human beings must use their free will to make decisions—and assume responsibility for those decisions—without knowing conclusively what is true or false, right or wrong, good or bad. In other words, there is no way of knowing absolutely what the correct choices are, and yet individuals must make choices all the time, and be held accountable for them. Sartre described this as a “terrifying freedom.” However, theologians such as American Paul Tillich (1886–1965) reconsidered the human condition in light of Christianity, arriving at far less pessimistic conclusions than did Sartre. For example, Tillich asserted that “divine answers” exist. Similarly, Jewish philosopher Martin Buber (1878–1965), who was also influenced by Kierkegaard, proposed that a personal and direct dialogue between the individual and God yields truths.