Here is implied the Aristotelian doctrine of habits, according to which a habit may be defined as 'a permanent acquired quality, resident in a power, and determining that power, originally indeterminate, to act regularly, readily, and pleasantly in a certain line of action.' Skill is a habit, so is virtue. The argument is that, as there are natural habits, so there should be a supernatural habit, which is 'habitual' or 'sanctifying' grace.

Of God and His Creatures: 3.151