We propose an account of the subject’s cognition and its relation to the world that allows for an articulation of the phenomenon of ideology. We argue that ideology is a form of what we call ‘a priori activity’: 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 relativize the a priori: we argue that (...) it is contingent, and therefore revisable. More precisely, it is conditioned materially, in that it is enmeshed with and shaped by material social practice. We conclude with some remarks about the possibility of agency over the relativized, materially conditioned a priori—that is, about the possibility of critique. (shrink)