That human beings sometimes act wrongly – be it through ignorance, weakness, or malice (deliberate wrongdoing) – is a commonplace of human experience; that divine grace can help them avoid such wrongdoing is a central feature of Christian doctrine, and thus accepted by all the Christian philosophers whose work is the principal focus of this chapter. These philosophers also accepted that the possibility of weakness – sometimes, but not always, characterized as weakness of will – results from a decisive sin of the first human beings. This “original sin” introduced the kind of disorder into human psychology that is, according to the medieval philosophers, the major component of moral weakness. Grace begins, among other things, the process of reordering this defective psychology. Two very disparate figures inform the presentation of these various issues in the high Middle Ages: Augustine and Aristotle. From Augustine, the medievals derived accounts of original sin, of grace, and – most importantly of all – the beginnings of a theory of consent somehow distinct from both reason and emotion; from Aristotle, they derived an account of moral weakness that they sometimes struggled to integrate with Augustinian teachings on the will, a view of the virtues that informed their account of grace, and a theory of motion that they used to talk about God’s activity in the soul.