St. Thomas, as usual, is the apostle of common sense. Virtue, like reason and language, is in us by nature potentially—we are designed for it—but since the actualization of this potentiality depends on our free choices, there is in us by our own choice both good and evil; and since choices make habits, there are in us both good and evil habits. (Virtues are simply good habits and vices are bad ones.)
–Peter Kreeft, Practical Theology: Spiritual Direction from St. Thomas Aquinas
