We propose an account of the subject’s cognition that allows for a full articulation of the phenomenon of ideology. We argue that ideology operates at the level of the a priori: it transcendentally conditions the intelligibility of thought and practice. But we draw from strands of post-Kantian philosophy of science and social philosophy in repudiating Kant’s view that the a priori is necessary and fixed. Instead, we argue, it is contingent, and therefore revisable. More precisely, it is conditioned materially: it (...) must be understood as an activity, continuous with and shaped by material social practice. We conclude with some remarks about the possibility of agency over one’s relativized, materially conditioned a priori; that is, over the possibility of critique. (shrink)