This book examines the concept of jouissance, a Lacanian term that refers to enjoyment experienced in different ways, from the enjoyment taken in an action that is ethically disapproved to the hidden pleasure taken by the patient in and from his symptom. Christian Fierens offers a new and rigorous explanation of jouissance as a third principle in the functioning of the unconscious, in addition to the technical and pleasure principles. The Jouissance Principle presents a detailed cross-reading of two key works: Kant’s Critique of Pure Reason and Lacan’s paper ‘Kant with Sade’, explaining how the functioning of the unconscious is a genuinely ethical process. The book also focuses on the role of psychoanalysis in relaunching the functioning of the unconscious, outlining the fourth form of Lacan’s object a and its stakes in the psychoanalytic process. An intriguing discussion of the relationship between pleasure, ethics and rationality, The Jouissance Principle will interest scholars of psychoanalysis and European philosophy, as well as helping clinicians to find a practical and ethical pathway through their practice.