Kant’s moral philosophy both enjoins the acquisition of self-knowledge as a duty, and precludes certain forms of its acquisition via what has become known as the Opacity Thesis. This article looks at several recent attempts to solve this difficulty and argues that they are inadequate. I argue instead that the Opacity Thesis rules out only the knowledge that one has acted from genuine moral principles, but does not apply in cases of moral failure. The duty of moral self-knowledge applies therefore only to one’s awareness of one’s status as a moral being and to the knowledge of one’s moral failings, both in particular actions and one’s overall character failings, one’s vices. This kind of knowledge is morally salutary as an aid to discovering one’s individual moral weakness as well as the subjective ends for which one acts, and in this way for taking up the morally required end of treating human beings as human beings. In this way, moral self-knowledge can be understood as a necessary element of moral improvement, and I conclude by suggesting several ways to understand it thereby as genuinely primary among the duties to oneself.