In this article, I present Aquinas's concept of sin in four different contexts precisely to highlight how theologically problematic and pastorally damaging his concept is in his early and later writings. First, I examine his account of the relationship between reason and the will in his early writings. In this context, sin was described as primarily an intellectual fault. Second, after 1270, he presented a new moral psychology that allowed him to distinguish charity from all moral virtues, intentions and actions. His concept of sin, however, remained substantially the same as in his earlier writings. Third, Aquinas's distinctions in moral psychology and among the virtues parallel the contemporary distinction between goodness (and badness) and rightness (and wrongness). But his teaching on sin, lacking the revisions he made in his psychology, does not. Finally, Josef Fuchs applies the contemporary distinction to his own writings on sin and develops a theologically correct notion, not found in Aquinas's writings. Thus, I investigate Aquinas's writings on sin as they stand against not only his earlier writings, but his later ones as well. Likewise I examine his thesis on sin against not only a contemporary distinction, but its specific application to sin as well. On all counts, major problems arise. I. THE WILL. REASON AND SIN IN THE EARLY WRITINGSThe will's freedom was central to scholastic theology: without freedom, there is no ground for merit or demerit. Yet, in his earlier writings, Aquinas's explanation of the will's freedom had two incompatible positions. On the one hand, he affirmed the will's autonomy: the will can move itself. On the other hand, when he explained how the will moves itself he always attributed the cause of that movement to reason.In the Commentary on the Sentences (In I1 Sen d38, ql,a2 ad5; a3 ad4; In 111 Sen d23,ql,ala; a2 ad3; In I11 Sen d27,q2,a4,so1.3~; In IV Sen d49,q3,a5,so13 ad2), in De veritate (14,5 ad5; 22,12c, and ad5), in the Summa Contra Gentiles (II,23 nr. 992; 47, nr. 1238; III,lO, nr. 1950; ~ 0 The EditorBasil Blackwell Ltd, Oxford, UK and Cambridge, USA.