The paper addresses the tension between Badiou’s claim that his theory of the subject must be considered first of all as a ‘formal’ theory, and a certain genealogical history of his notion of the subject. In the latter case, it seems that a very specific political experience has played a crucial role (at least) for Badiou in his early conception of the subject. More particularly, the paper addresses this tension from an ethical perspective. As for the claim repeatedly made in his work, one can identify an implicitly ethical disposition in the formalization itself. At the same time, there are several formulations in his writings that seem to exceed the formal level. The paper examines four concepts or formulations appearing in his three main books (Theory of the Subject, Being and Event, Logics of Worlds) that seem to express a more or less explicit ethical dimension, namely his theory of affects, the principle ‘to decide the undecidable’, the contrast between ‘fidelity’ and ‘confidence’ and Badiou’s answer to the question What is it to live? The paper’s aim is to pinpoint the difference between the ethical stance implied in the formal description of Badiou’s theory of the subject and an explicit ‘ethics of the subject’. The author’s hypothesis is that the latter embodies a dimension that remains tacit in the formal expression of the subject’s ‘household’ alone: Badiou’s ubjectivity itself.