This article traces the consequences for Badiou's political subjectivity if his understanding of the Cartesian subject is incorrect. For Badiou, the faithful subject, political and otherwise, is formed through fidelity to the appearance of an event of truth, and the process of this fidelity creates a world. These truths are immanent to the worlds in which they appear. An obscure subject, however, is faithful to a negation, while a reactive subject denies the appearance of a truth's event. Badiou's subject radicalizes Lacan's radicalization of the Cartesian subject, but for him both Descartes and Lacan consider the subject stable since they are caused by truth rather than by the event of a truth. However, immanent to Descartes's philosophy is an unstable subject, thanks to the role of the imagination in the discovery of the cogito. Fidelity to this immanent Cartesian subject shows Lacan as an obscure subject and Badiou as reactive.