Chapter 2 argues that Thomas’s mature view of the formal cause of original justice created an unresolved problem for his doctrine of original sin. Though his early writings sharply distinguished the rectitude of the human will in the state of original justice from supernatural sanctifying grace, by the mid-1260s (e.g., STh I, q. 95, a. 1) he implied that the formal cause of original justice is sanctifying grace. The problem is that Thomas also held (1) that Adam should have been the principal cause of original justice in his posterity and (2) that no creature can be the principal cause of sanctifying grace. Thomas’s mature view implies that the disposition to original justice never could have been sexually transmitted. This implies that his account of original sin as a whole needed to be modified. Adam’s failure to transmit the disposition to original justice rendered the lack of original justice sinful in his posterity: if Adam couldn’t have done this in the first place, how could his descendants have original sin?