In the first decades of the 20th century, a number of philosophers and writers explored existentialist ideas. The
Spanish philosopher Miguel de Unamuno y Jugo, in his 1913 book The Tragic Sense of Life in Men and Nations,
emphasized the life of "flesh and bone" as opposed to that of abstract rationalism. Unamuno rejected systematic
philosophy in favor of the individual's quest for faith. He retained a sense of the tragic, even absurd nature of the
quest, symbolized by his enduring interest in Cervantes' fictional character Don Quixote. A novelist, poet and
dramatist as well as philosophy professor at the University of Salamanca, Unamuno wrote a short story about a
priest's crisis of faith, Saint Manuel the Good, Martyr, which has been collected in anthologies of existentialist
fiction. Another Spanish thinker, Ortega y Gasset, writing in 1914, held that human existence must always be
defined as the individual person combined with the concrete circumstances of his life: "Yo soy yo y mis
circunstancias" ("I am myself and my circumstances"). Sartre likewise believed that human existence is not an
abstract matter, but is always situated, also many thought his plays were absurd ("en situación").
Although Martin Buber wrote his major philosophical works in German, and studied and taught at the Universities of
Berlin and Frankfurt, he stands apart from the mainstream of German philosophy. Born into a Jewish family in
Vienna in 1878, he was also a scholar of Jewish culture and involved at various times in Zionism and Hasidism. In
1938, he moved permanently to Jerusalem. His best-known philosophical work was the short book I and Thou,
published in 1922. For Buber, the fundamental fact of human existence, too readily overlooked by scientific
rationalism and abstract philosophical thought, is "man with man", a dialogue that takes place in the so-called
"sphere of between" ("das Zwischenmenschliche"). Two Ukrainian/Russian thinkers, Lev Shestov and Nikolai Berdyaev, became well known as existentialist thinkers
during their post-Revolutionary exiles in Paris. Shestov, born into a Ukrainian-Jewish family in Kiev, had launched
an attack on rationalism and systematization in philosophy as early as 1905 in his book of aphorisms All Things Are
Possible.
Berdyaev, also from Kiev but with a background in the Eastern Orthodox Church, drew a radical distinction between
the world of spirit and the everyday world of objects. Human freedom, for Berdyaev, is rooted in the realm of spirit,
a realm independent of scientific notions of causation. To the extent the individual human being lives in the objective
world, he is estranged from authentic spiritual freedom. "Man" is not to be interpreted naturalistically, but as a being
created in God's image, an originator of free, creative acts. He published a major work on these themes, The
Destiny of Man, in 1931.
Gabriel Marcel, long before coining the term "existentialism", introduced important existentialist themes to a French
audience in his early essay "Existence and Objectivity" (1925) and in his Metaphysical Journal (1927). A
dramatist as well as a philosopher, Marcel found his philosophical starting point in a condition of metaphysical
alienation: the human individual searching for harmony in a transient life. Harmony, for Marcel, was to be sought
through "secondary reflection", a "dialogical" rather than "dialectical" approach to the world, characterized by
"wonder and astonishment" and open to the "presence" of other people and of God rather than merely to
"information" about them. For Marcel, such presence implied more than simply being there (as one thing might be in the presence of another thing); it connoted "extravagant" availability, and the willingness to put oneself at the
disposal of the other. Marcel contrasted secondary reflection with abstract, scientific-technical primary reflection, which he associated
with the activity of the abstract Cartesian ego. For Marcel, philosophy was a concrete activity undertaken by a
sensing, feeling human being incarnate — embodied — in a concrete world. Although Jean-Paul Sartre
adopted the term "existentialism" for his own philosophy in the 1940s, Marcel's thought has been described as
"almost diametrically opposed" to that of Sartre.[38] Unlike Sartre, Marcel was a Christian, and became a Catholic
convert in 1929.
In Germany, the psychologist and philosopher Karl Jaspers — who later described existentialism as a "phantom"
created by the public — called his own thought, heavily influenced by Kierkegaard and Nietzsche,
Existenzphilosophie. For Jaspers, "Existenz-philosophy is the way of thought by means of which man seeks to
become himself...This way of thought does not cognize objects, but elucidates and makes actual the being of the
thinker." Jaspers, a professor at the University of Heidelberg, was acquainted with Martin Heidegger, who held a
professorship at Marburg before acceding to Edmund Husserl's chair at Freiburg in 1928. They held many philosophical
discussions, but later became estranged over Heidegger's support of National Socialism. They shared an admiration
for Kierkegaard,[ and in the 1930s, Heidegger lectured extensively on Nietzsche. Nevertheless, the extent to which
Heidegger should be considered an existentialist is debatable. In Being and Time he presented a method of rooting
philosophical explanations in human existence (Dasein) to be analysed in terms of existential categories
(existentiale); and this has led many commentators to treat him as an important figure in the existentialist movement