This introductory course in the Sustainability line seeks to break with common sense about what it means to be "sustainable". Based on a presentation of the main sustainability paradigms (weak and strong), it proposes a critical reflection on the ethical, economic and epistemic limits of the dominant development model. The multiple dimensions of the current civilizational crisis are explored, with an emphasis on planetary limits and the interdependence between ecological collapse and social inequalities. Edgar Morin's complex thinking is introduced as a tool for dealing with uncertainty and promoting thought reform. The course also presents Socio-ecological Systems as an integrative approach to evaluating and transforming realities, and invites students to learn about and imagine systemic alternatives, based on plural knowledge and counter-hegemonic experiences expressed in the Pluriverse. In the end, it proposes an exercise in unlearning and relearning, with a focus on building possible futures and the ethical positioning of students as agents of transition.