This work aims to show the efficiency and operative capacity of the concept of separation when used to consider early relations, including the period of primary narcissism and fusion to the object, as well as their readjustment at adolescence. The limits of the concept of the separation-individuation process, proposed by M. Mahler and taken up again by P. Blos, are questioned because of the teleological character of their conception of the human psyche. The case of an adolescent girl who has difficulty saying « I », makes it possible to distinguish processes of differentiation and processes of subjectivation, and to show that separation plays a crucial role in these two processes. However, in a concomitant way, a unitary dream of non-separation, close to the myth of the androgyne such as it was related in Plato’s Banquet, runs through psychic life.