The starting point for Luther's 95 theses on indulgence, which - unforeseen by the author - launched the Reformation, was a new understanding of penitence, and indeed this was something he focussed on during the early years of the Reformation, e.g., in Ein Sermon vom Sakrament der Buße. Still, this is arguably a sidetrack in Luther's theological development. Penitence was not an important topic in the disputations from 1515 to 1518, which are considered landmark documents in Luther's theological development, and was not at all touched in De servo arbitrio, which Luther later considered his most important work. So why this sudden interest in penitence, and what is the relation between this interest and the structuring metaphysics of Luther's thought?