Martin Heidegger (September 26, 1889 – May 26, 1976) was a German philosopher known for his existential and phenomenological explorations of the "question of Being."
His central belief was that philosophy, and society as a whole, was preoccupied with what it is that exists. His belief was that we find ourselves "always already" fallen into a world that already existed. But he insisted that we had forgotten the basic question of what it is to exist, of what being itself is. This question defines our central nature. He argued that we are practical agents, caring and concerned about our projects in the world, and allowing it to reveal, or 'unconceal' itself to us. He came to believe that our proactive interference and manipulation of reality is often harmful and hides our true being as essentially limited participants, not masters, of the world which we discover.
Heidegger wrote about these issues in his best-known book, Being and Time (1927), which is considered to be one of the most important philosophical works of the 20th century. Heidegger's views have implications beyond philosophy, in literature, psychology, theology and artificial intelligence.
He remains controversial due to his membership in the Nazi Party and statements in support of Adolf Hitler, for which he never apologized or expressed regret.
Source: Wikipedia, "Martin Heidegger", available under the CC-BY-SA License.