Critical thinking (CT) is frequently mentioned as a key competence in sustainability curricula. In this context our era is often diagnosed as being 'post-truth', indicating an epistemic concern. However, emerging 'post-sustainable' views in education indicate that environmental crises are posing increasingly existential concerns, which might partly explain why simple consciousness-raising sometimes faces denial or fails to promote sustainable action. To overcome this challenge, we undertake a philosophical critique of modern (individual, rational, autonomous) subjectivity assumed in CT and much of curricular thinking. We follow the 'ontological turn' where criticality means self-reflective questioning of one's own being-in-the-world. One acute question concerns energy, especially fossil fuels, which constitute much of the autonomous experience of modern, critical subjectivity, while simultaneously endangering the future horizon of that same subjectivity. Climate strikes at schools and the yellow vest movements indicate, in their own ways, how ecologically problematic fossil fuels are bending modern rationality into unpredicted directions. Metaphoric Archimedes and his 'circles in the sand' demonstrate the vulnerability of critical thought facing post-sustainability. This vulnerability should be addressed in curriculum theory, since it is interdependent persons--rather than independent subjects--who are open to sustainable transformation and action.