Descartes is widely portrayed as the arch proponent of “the epistemological transparency of thought” (or simply, “Transparency”). The most promising version of this view—Transparency-through-Introspection—says that introspecting (i.e., inwardly attending to) a thought guarantees certain knowledge of that thought. But Descartes rejects this view and provides numerous counterexamples to it. I argue that, instead, Descartes’s theory of self-knowledge is just an application of his general theory of knowledge. According to his general theory, certain knowledge is acquired only through clear and distinct intellection. Thus, in his view, certain knowledge of one’s thoughts is acquired only through clear and distinct intellection of one’s thoughts. Introspection is a form of intellection and it can be clear and distinct. Ordinarily, however, introspection isn’t clear and distinct but is instead confused with dubitable perceptions of bodies. To make introspection clear and distinct, we need to “sharply separate” it from all perceptions of bodies by doubting all perceptions of bodies. Without such radical doubt, introspection remains confused and we lack certain knowledge not just of the specific features of our thoughts, but even of the minimal claim that a thought exists. Far from being the high priest of Transparency, Descartes is radically opposed to it.