There are always third parties that expand, limit, and regulate the relationship between two. The author consistently applies this approach to the entire development of humans and presents a psychology of the third and an architecture of the mental space. The symbolic structures of culture are also taken into account in their triangulating function and practical significance, which play an important role in the establishment of mental space and the delineation of inside and outside. Through numerous examples, it is demonstrated how this triadic perspective expands the possibilities of perception and action in counseling, psychotherapy, and medical practice. The search for the third also allows for progressive steps in blocked situations and therapeutic dead ends.