Martin Heidegger was a German philosopher known for his existential and phenomenological explorations of the "question of Being".
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. But philosophy, Western Civilization's chief way of questioning, had in the process of philosophizing 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.
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. 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 neither apologized nor expressed regret, except in private when he called it "the biggest stupidity of his life". The so-called Heidegger controversy raises general questions about the relation between Heidegger's thought and his connection to National Socialism.