Scotland's philosophers of the medieval period, priests almost to a man, were deeply interested in the concept of sin. The concept resonates with philosophical overtones, and our early philosophers found something philosophical to say about it. The greatest of those philosophers, John Duns Scotus, wrote extensively on sin in the course of his Commentary on the Sentences of Peter Lombard. Lombard quotes Jerome's dictum that there is sin in thought, word and deed, and in his Commentary on the Sentences Duns Scotus probes this dictum, since it is not only central to moral theology but also problematic to philosophy. I shall attend to an aspect of Scotus's investigation, that concerning the relation between will and sinful thinking. I shall argue against one of his theses and shall seek to replace it with one which is in a variety of ways more defensible.