Heidegger's central belief was that philosophy, and society as a whole, was preoccupied with what it is that exists. 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.
He wrote about many of these issues in his best-known book, Being and Time. This is considered to be one of the most important philosophical works of the 20th century and he has been influential beyond philosophy, in literature, psychology, and artificial intelligence.
Heidegger remains controversial due to his membership of the Nazi Party and statements in support of Adolf Hitler.