Martin Heidegger is considered by some scholars to be the foremost philosopher of the twentieth century. He was born in Messkirch, Germany, in 1889, attended schools in the area, and for a short time considered becoming a priest. But he turned instead to philosophical studies, primarily at the university in Freiburg, near his home town. He married Elfride Petri in 1917, and they reared two sons.

After the First World War, Heidegger began to teach courses in Freiburg, and later at Marburg, that attracted the attention of gifted students, among them Karl Löwith, Jacob Klein, Hans Jonas, Hannah Arendt, Herbert Marcuse, Leo Strauss, and Hans-Georg Gadamer. These courses included new interpretations of Aristotle and Augustine, and analyses of everyday life and experience. His lectures were issued in 1927 in his first, and still most important, major publication, Sein und Zeit (Being and Time). In 1929 he published Kant and the Problem of Metaphysics.

Heidegger had left Freiburg in 1923 to teach in Marburg, but he returned to Freiburg in 1928 to replace his teacher, Edmund Husserl, the founder of phenomenology. In 1933 the Nazi regime appointed him rector of Freiburg, and he joined the Nazi party. He left the rectorate after ten months but remained a party member and was associated with many of those who gave intellectual support to the regime. He continued to teach and lecture during the 1930s and 1940s, but his association with the Nazis led the occupying authorities to bar him from teaching after the end of World War II.

Heidegger remained in Germany and began to lecture and teach again in 1951. In the post-war years, his intellectual influence spread to France, the United States, and beyond. He published several collections of essays, works on technology, thought, and the principle of identity, and material from courses that he had given on Nietzsche and various aspects of metaphysics. Being and Time became a major source for the understanding of existentialism, a philosophic movement that was growing in importance and popularity among academics and intellectuals. Existentialist thinkers influenced by Heidegger included Jean-Paul Sartre and Simone de Beauvoir.

Heidegger’s association with the Nazis has never ceased to be a source of controversy, although scholars’ and journalists’ understanding of the depth and degree of this connection, and its relation to his thought, has differed in different periods. Heidegger discusses this issue most fully in an interview he gave to the German magazine Der Spiegel in 1966, published after his death. Recent evidence—particularly the so-called “Black Notebooks”—displays a deeper connection than he suggested there.

After Heidegger’s death in 1976, courses he taught, manuscripts he prepared, and notebooks he kept began to be published, a process that still continues. They offer evidence of the sources of Being and Time, his novel understanding of great thinkers of the past, and changes in the emphases and direction of his thought that began in the 1930s.