The author shows that Aquinas's commentary interprets Aristotle's remark about the destruction of virtue correctly. The author discusses Aquinas's concept of the will as a capacity for free choice: a notion central to his conviction that people can lose their virtue. The author considers two parts of his Ethics commentary where Aquinas clearly injects his own opinion: that virtues divide into principal and merely secondary virtues, and that disposition is something we exercise when we will. Aquinas's conviction that virtuous people can lose their virtue through moral backsliding carries with it a more cheering corollary: with enough time and effort, vicious people can improve. In both cases the individual's power of free choice gives her the capacity to change her character. As Aquinas's ethical theory denies that anyone on earth is infallibly virtuous, so it denies that anyone on earth is incurably vicious.