A psychological understanding of interpersonal processes in terms of complementarity is not new. It is enough to mention Buber (the title of our paper refers to an expression of his), as well as Bateson and his definitions of double description, binocular vision and complementary and symmetric relations. We would like to clarify the nature of complementarity, and to point out the presence of this framework in some philosophical and scientific discourses about the person. Moreover, we think that the adoption of a framework of complementarity becomes a metaphysical necessity within what we have called “hermeneutic constructivism,” and that other constructivisms fail to acknowledge it, thereby losing much of their metatheoretical, revolutionary potential. We will document the possibility of adopting a framework of complementarity with respect to different pairs of poles, which specify as many phenomenal domains: (1) the relation between any entity and its environment; (2) the relation between modes of description; (3) the relation between the person and the world; and (4) the relation between people. In the final part of the paper we outline some implications of a consideration of complementarity for the psychotherapy process. Relevance: The framework of complementarity is an essential feature of hermeneutic constructivism.