THEO 20605 Lecture Notes: Conscience and Truth
Now for Aquinas, virtue and law are the internal and external sides (or standards) in the same pursuit of goodness. For Aquinas and those who follow him in the tradition of Catholic moral thought, there can be no ultimate conflict between what the law requires and what virtue trains us to do. In other words, people who are truly virtuous abide by the law of God; and adherence to just laws (those human laws that accord with the natural law) helps to form persons in virtuous character.
Law he calls an external principle of act (a standard the guides us from the outside, just as the habit of virtue guides us from the inside)