Martin Heidegger ( ; September 26, 1889 – May 26, 1976) was a German philosopher known for his existential and phenomenological explorations of the "question of Being".

Heidegger maintained that philosophy, in the process of philosophizing, 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". Our way of questioning defines our nature. He advocated a return to the practical being in the world, allowing it to reveal, or "unconceal" itself as concealment.

Writing extensively on Nietzsche in his later career, and offering a "phenomenological critique of Kant" in his Kant and the Problem of Metaphysics, Heidegger is known for his post-Kantian philosophy. His best known book, Being and Time is considered one of the most important philosophical works of the 20th century. Heidegger's influence has been far reaching, from philosophy to theology, deconstructionism, literary theory, architecture, and artificial intelligence.

Heidegger is a controversial figure, largely for his affiliation with Nazism, for which he never apologized nor expressed regret, except in private when he called it "the biggest stupidity of his life" (die größte Dummheit seines Lebens). The so-called Heidegger controversy raises general questions about the relation between Heidegger's thought and his connection to National Socialism.

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