A clinic of limits is essential; it concerns not only the limits between structures but also what makes a limit for a subject, and it assumes the analyst’s awareness of the limits he should not encourage the subject to go beyond. For this reason the authors make a detour on the question of the mark. Jacques Lacan first addresses the notion of the mark via the unary trait, the primary signifier for the subject. Secondly, the mark is approached via the effects of jouissance. In the seminar The Four Fundamental Concepts of Psychoanalysis Lacan considers what is at stake, namely, the way the unconscious touches the sexual. The formations of the unconscious constitute the dimension without which diagnosis would not be anything other than the name of a group of symptoms, and psychoanalysis just a complement of psychiatry.