This article considers a central ethically relevant interpersonal emotion, guilt. It is argued that guilt, as an irreducible moral category, has a constitutive role to play in our ways of conceptualizing our relations to other people. Without experiencing guilt, or being able to do so, we would not be capable of employing the moral concepts and judgments we do employ. Elaborating on this argument, the paper deals with what may be described as the “metaphysics of guilt.” More generally, it is suggested, through a case study on the concept of guilt, that a moral theory avoiding naïve emotivism yet emphasizing the role of emotions in morality can and should pay attention to the transcendental status of emotions such as guilt—emotions constitutive of our concept of moral seriousness. Instead of psychologizing moral emotions, the paper employs Raimond Gaita's Wittgenstein-inspired way of examining the place of the concepts of guilt and remorse in our ethical language-use. Finally, some methodological remarks on the possibility of transcendental reflection in moral philosophy are presented. While it is not necessary to commit oneself to any specific religious tradition in order to emphasize the constitutive role of guilt in the way suggested in the paper, it turns out that the moral depth of this concept requires that one is at least open to religiously relevant ways of using moral language. In the fundamental metaphysical sense examined in the paper, guilt is a concept whose home language-game is religious rather than secular ethics.