Martin Heidegger (German: [ˈmaɐ̯tiːn ˈhaɪdɛɡɐ]; September 26, 1889 – May 26, 1976) was a German philosopher known for his existential and phenomenological explorations of the "question of Being". Heidegger is known for offering a phenomenological critique of Kant. He wrote extensively on Nietzsche and Hölderlin in his later career. Heidegger's influence has been far reaching, influencing fields such as philosophy, theology, art, architecture, artificial intelligence, cultural anthropology, literary theory, social theory, political theory, psychiatry, and psychotherapy.
His best known book, Being and Time, is considered one of the most important philosophical works of the 20th century. In it and later works, Heidegger maintained that our way of questioning defines our nature. He argued that philosophy, Western civilization's chief way of questioning, had lost sight of the being it sought. Finding ourselves "always already" fallen in a world of presuppositions, we lose touch with what being was before its truth became "muddled". As a solution to this condition, Heidegger advocated a return to the practical being in the world, allowing it to reveal, or "unconceal" itself as concealment.
Heidegger is a controversial figure, largely for his affiliation with Nazism prior to 1934, for which he neither apologized nor expressed regret, except in private when he called it "the biggest stupidity of his life" (die größte Dummheit seines Lebens). The controversy raises general questions about the relation between Heidegger's thought and his connection to National Socialism.