Objection 1. It would seem that the definition, usually given, of virtue, is not suitable, to wit: "Virtue is a good quality of the mind, by which we live righteously, of which no one can make bad use, which God works in us, without us." For virtue is man's goodness, since virtue it is that makes its subject good. But goodness does not seem to be good, as neither is whiteness white. It is therefore unsuitable to describe virtue as a "good quality."
Objection 2. Further, no difference is more common than its genus; since it is that which divides the genus. But good is more common than quality, since it is convertible with being. Therefore "good" should not be put in the definition of virtue, as a difference of quality.
Objection 3. Further, as Augustine says (De Trin. xii, 3): "When we come across anything that is not common to us and the beasts of the field, it is something appertaining to the mind." But there are virtues even of the irrational parts; as the Philosopher says (Ethic. iii, 10). Every virtue, therefore, is not a good quality "of the mind."
Objection 4. Further, righteousness seems to belong to justice; whence the righteous are called just. But justice is a species of virtue. It is therefore unsuitable to put "righteous" in the definition of virtue, when we say that virtue is that "by which we live righteously."
Objection 5. Further, whoever is proud of a thing, makes bad use of it. But many are proud of virtue, for Augustine says in his Rule, that "pride lies in wait for good works in order to slay them." It is untrue, therefore, "that no one can make bad use of virtue."
Objection 6. Further, man is justified by virtue. But Augustine commenting on John 15:11: "He shall do greater things than these," says [Tract. xxvii in Joan.: Serm. xv de Verb. Ap. 11: "He who created thee without thee, will not justify thee without thee." It is therefore unsuitable to say that "God works virtue in us, without us."
On the contrary, We have the authority of Augustine from whose words this definition is gathered, and principally in De Libero Arbitrio ii, 19.
I answer that, This definition comprises perfectly the whole essential notion of virtue. For the perfect essential notion of anything is gathered from all its causes. Now the above definition comprises all the causes of virtue. For the formal cause of virtue, as of everything, is gathered from its genus and difference, when it is defined as "a good quality": for "quality" is the genus of virtue, and the difference, "good." But the definition would be more suitable if for "quality" we substitute "habit," which is the proximate genus.
Now virtue has no matter "out of which" it is formed, as neither has any other accident; but it has matter "about which" it is concerned, and matter "in which" it exits, namely, the subject. The matter about which virtue is concerned is its object, and this could not be included in the above definition, because the object fixes the virtue to a certain species, and here we are giving the definition of virtue in general. And so for material cause we have the subject, which is mentioned when we say that virtue is a good quality "of the mind."
The end of virtue, since it is an operative habit, is operation. But it must be observed that some operative habits are always referred to evil, as vicious habits: others are sometimes referred to good, sometimes to evil; for instance, opinion is referred both to the true and to the untrue: whereas virtue is a habit which is always referred to good: and so the distinction of virtue from those habits which are always referred to evil, is expressed in the words "by which we live righteously": and its distinction from those habits which are sometimes directed unto good, sometimes unto evil, in the words, "of which no one makes bad use."
Lastly, God is the efficient cause of infused virtue, to which this definition applies; and this is expressed in the words "which God works in us without us." If we omit this phrase, the remainder of the definition will apply to all virtues in general, whether acquired or infused.
Reply to Objection 1. That which is first seized by the intellect is being: wherefore everything that we apprehend we consider as being, and consequently as gone, and as good, which are convertible with being. Wherefore we say that essence is being and is one and is good; and that oneness is being and one and good: and in like manner goodness. But this is not the case with specific forms, as whiteness and health; for everything that we apprehend, is not apprehended with the notion of white and healthy. We must, however, observe that, as accidents and non-subsistent forms are called beings, not as if they themselves had being, but because things are by them; so also are they called good or one, not by some distinct goodness or oneness, but because by them something is good or one. So also is virtue called good, because by it something is good.
Reply to Objection 2. Good, which is put in the definition of virtue, is not good in general which is convertible with being, and which extends further than quality, but the good as fixed by reason, with regard to which Dionysius says (Div. Nom. iv) "that the good of the soul is to be in accord with reason."
Reply to Objection 3. Virtue cannot be in the irrational part of the soul, except in so far as this participates in the reason (Ethic. i, 13). And therefore reason, or the mind, is the proper subject of virtue.
Reply to Objection 4. Justice has a righteousness of its own by which it puts those outward things right which come into human use, and are the proper matter of justice, as we shall show further on (60, 2; II-II, 58, 8). But the righteousness which denotes order to a due end and to the Divine law, which is the rule of the human will, as stated above (Question 19, Article 4), is common to all virtues.
Reply to Objection 5. One can make bad use of a virtue objectively, for instance by having evil thoughts about a virtue, e.g. by hating it, or by being proud of it: but one cannot make bad use of virtue as principle of action, so that an act of virtue be evil.
Reply to Objection 6. Infused virtue is caused in us by God without any action on our part, but not without our consent. This is the sense of the words, "which God works in us without us." As to those things which are done by us, God causes them in us, yet not without action on our part, for He works in every will and in every nature.
Source: Summa Theologica (New Advent)