Existentialism is a philosophical movement that rejects any predetermined role for human beings. Unlike tools, which are designed in order to fill some preconceived role (for example, a knife's preconceived role, or essence, is to cut), human beings are capable, to some extent at least, of deciding for themselves what constitutes their own essence. Although they didn't use the term, the nineteenth-century philosophers Søren Kierkegaard and Friedrich Nietzsche are widely regarded as the fathers of existentialism. Their influence, however, has extended beyond existentialist thought. Religious thinkers were among those influenced by Kierkegaard. Christian existentialists include Gabriel Marcel, Nicholas Berdyaev, Miguel de Unamuno, and Karl Jaspers. The Jewish philosophers Martin Buber and Lev Shestov have also been associated with existentialism.
Two of the targets of Kierkegaard and Nietzsche's writings were the philosophical systems of Hegel and Schopenhauer respectively, which they had each admired in their youths. Kierkegaard thought Hegel ignored or excluded the inner subjective life of living human beings, while Nietzsche thought Schopenhauer's pessimism led people to live an ascetic, or self-hating, life. Kierkegaard suggested that “truth is subjectivity,” arguing that what is most important to a living individual are questions dealing with one's inner relationship to life. Nietzsche proposed perspectivism, which is the view that truth depends on individual perspectives.
Influence notably by Kierkegaard, the German philosopher Martin Heidegger is generally considered an existentialist thinker and one of the key figures in 20th century thought. In Being and Time (1927), he presented a method of rooting philosophical explanations in human existence (Dasein) to be analysed in terms of existential categories. In The Letter on Humanism, however, Heidegger explicitly rejected the existentialism of Jean-Paul Sartre. Karl Jaspers is another important German existentialist philosopher.
Sartre became the best-known proponent of existentialism, exploring it not only in theoretical works such as Being and Nothingness, but also in plays and novels. Sartre, along with Albert Camus, Maurice Merleau-Ponty and Simone de Beauvoir, all represented an avowedly atheistic branch of existentialism, which is now more closely associated with their ideas of the novel nausea, contingency, bad faith, and the absurd than with Kierkegaard's spiritual angst. Nevertheless, the focus on the individual human being, responsible before the universe for the authenticity of his or her existence, is common to all these thinkers.