For more than ten years, psychiatrist and proponent of "medical Dasein-analysis" Medard Boss organized seminars in Zollikon, Switzerland, where philosopher Martin Heidegger analyzed for psychiatrists and psychotherapists the issue of the ontological foundation of their science.

According to the philosopher, psychiatry and psychotherapy are based on inappropriate ontological foundations, which are uncritically adopted merely through the application of the natural scientific method to the phenomena of the human psyche. The search for appropriate ontological foundations and methodological tools requires, on the contrary, a genuine phenomenological approach.

These issues are examined by the philosopher not only in the seminars themselves (Part A, ed. Roes, 2015) but also in his personal conversations with Boss and in his letters to him. He offers here often enlightening explanations and additions to what he discusses in the seminars.

In this personal communication with Boss, he faces questions concerning specific issues of psychiatry and psychotherapy. He directly addresses topics such as projection and introjection, dreams and their interpretation, Freud's psychic components of "It, Ego, and Superego," among others.

With the conversations and the letters, the Greek edition of the so-called Zollikon Seminars is completed. The texts included in them hold a special position in Heidegger's work. They are the only text of his that deals so thoroughly with the philosophical foundation of a science and articulate his critique of contemporary science and its absolutization.

They also offer access to his reflection for those interested without specific philosophical knowledge – as the philosopher, in conversation with non-specialists here, uses language that is much simpler and more understandable than in his other writings.