Karl Jaspers was a German philosopher whose place in the history of philosophy is based primarily on his work in existentialism indeed, he may be considered the first to construct a self-consciously existentialist philosophy in the modern era. Jaspers originally studied law and medicine, even receiving an M.D. from University of Heidelberg and teaching psychiatry at Heidelberg for a while. He probably could have had a distinguished career in psychiatry had he not later turned to philosophy.
According to Jaspers, philosophy was not a matter of any particular conclusions on any particular set of issues analyzed through abstract systems, but rather a specific type of thinking about the way the world is and how the world works. An important concept in his works is that of das Umgreifende (the encompassing), which he used to refer to the ultimate limits of being, an indefinite horizon in which both subjective and objective experience is made possible, but which we cannot understand rationally and can never actually see.
Another key concept for Jaspers was that of Existenz (existence, although it doesnt really mean quite the same as the English). He used this to describe the state of freedom and possibility for authentic being of individuals who have become consciously aware of the encompassing and confront limiting situations in human life like guilt, conflict, and even death. Reason may create the boundaries for contemplating the objects in life, but Existenz creates the boundaries for contemplating the personal subject which does the contemplating.
All of this, according to Jaspers, has been overshadowed in the modern era by science, technology, the loosening of religious bonds, and the development of mass political movements. He hoped that the creation of a new philosophy that acknowledged the central role of humanity might serve to overcome such problems and eventually restore to us a better sense of who we are and our place in the world.
Critical to achieving this, Jaspers thought, was developing a better awareness of the Transcendent that which traditional theology labels God. The Transcendent is pure personal experience, something we can become aware of as we also become aware of our finite natures. Awareness of the Transcendent produces awareness of the radical freedom in each person the freedom to choose, the freedom to decide, and most of all the freedom to commit oneself to a particular course of action that brings meaning and purpose to life.
In this, Jaspers echoes the ideas of Kierkegaard where he emphasized the importance of a leap of faith which transcends rational, objective considerations. Jaspers, though, labeled this a philosophic faith because he no longer relied upon the traditional religious categories which still animated Kierkegaards writings. They shared, however, the basic idea that a person is ultimately faced with an either-or decision without the aid of objective proof or knowledge about what the right choice might be.