Some therapeutic approaches developed out of the European school of existential philosophy. Concerned mainly with the individual's ability to develop and preserve a sense of meaning and purpose throughout life, major contributors to the field (e.g., Irvin Yalom, Rollo May) and Europe (Viktor Frankl, Ludwig Binswanger, Medard Boss, R.D.Laing, Emmy van Deurzen) attempted to create therapies sensitive to common "life crises" springing from the essential bleakness of human self-awareness, previously accessible only through the complex writings of existential philosophers (e.g., Søren Kierkegaard, Jean-Paul Sartre, Gabriel Marcel, Martin Heidegger, Friedrich Nietzsche). The uniqueness of the patient-therapist relationship thus also forms a vehicle for therapeutic inquiry. A related body of thought in psychotherapy started in the 1950s with Carl Rogers. Based also on the works of Abraham Maslow and his hierarchy of human needs, Rogers brought person-centered psychotherapy into mainstream focus. The primary requirement was that the client receive three core "conditions" from his counselor or therapist: unconditional positive regard, sometimes described as "prizing" the client's humanity; congruence [authenticity/genuineness/transparency]; and empathic understanding. This type of interaction was thought to enable clients to fully experience and express themselves, and thus develop according to their innate potential. Others developed the approach, like Fritz and Laura Perls in the creation of Gestalt therapy, as well as Marshall Rosenberg, founder of Nonviolent Communication, and Eric Berne, founder of transactional analysis. Later these fields of psychotherapy would become what is known as humanistic psychotherapy today. Self-help groups and books became widespread.