The discussions among philosophers about the comparison between what is virtuous and what is useful have nothing to do with Christians. For with them nothing is useful which is not just. What are the duties of perfection, and what are ordinary duties? The same words often suit different things in different ways. Lastly, a just man never seeks his own advantage at the cost of another's disadvantage, but rather is always on the lookout for what is useful to others.
8. As we have already spoken about the two former subjects, wherein we discussed what is virtuous and what is useful, there follows now the question whether we ought to compare what is virtuous and useful together, and to ask which we must follow. For, as we have already discussed the matter as to whether a thing is virtuous or wicked, and in another place whether it is useful or useless, so here some think we ought to find out whether a thing is virtuous or useful.
9. I am induced to do this, lest I should seem to be allowing that these two are mutually opposed to one another, when I have already shown them to be one. For I said that nothing can be virtuous but what is useful, and nothing can be useful but what is virtuous. For we do not follow the wisdom of the flesh, whereby the usefulness that consists in an abundance of money is held to be of most value, but we follow that wisdom which is of God, whereby those things which are greatly valued in this world are counted but as loss.
10. For this χατόρθωμα, which is duty carried out entirely and in perfection, starts from the true source of virtue. On this follows another, or ordinary duty. This shows by its name that no hard or extraordinary practice of virtue is involved, for it can be common to very many. The desire to save money is the usual practice with many. To enjoy a well-prepared banquet and a pleasant meal is a general habit; but to fast or to use self-restraint is the practice of but few, and not to be desirous of another's goods is a virtue rarely found. On the other hand, to wish to deprive another of his property— and not to be content with one's due— here one will find many to keep company with one. Those (the philosopher would say) are primary duties— these ordinary. The primary are found but with few, the ordinary with the many.
11. Again, the same words often have a different meaning. For instance, we call God good and a man good; but it bears in each case quite a different meaning. We call God just in one sense, man in another. So, too, there is a difference in meaning when we call God wise and a man wise. This we are taught in the Gospel: “Be perfect even as your Father Who is in heaven is perfect.” I read again that Paul was perfect and yet not perfect. For when he said: “Not as though I had already attained, either were already perfect; but I follow after, if that I may apprehend it.” Immediately he added: “We, then, that are perfect.” There is a twofold form of perfection, the one having but ordinary, the other the highest worth. The one availing here, the other hereafter. The one in accordance with human powers, the other with the perfection of the world to come. But God is just through all, wise above all, perfect in all.
12. There is also diversity even among men themselves. Daniel, of whom it was said: “Who is wiser than Daniel?” was wise in a different sense to what others are. The same may be said of Solomon, who was filled with wisdom, above all the wisdom of the ancients, and more than all the wise men of Egypt. To be wise as men are in general is quite a different thing to being really wise. He who is ordinarily wise is wise for temporal matters, is wise for himself, so as to deprive another of something and get it for himself. He who is really wise does not know how to regard his own advantage, but looks with all his desire to that which is eternal, and to that which is seemly and virtuous, seeking not what is useful for himself, but for all.
13. Let this, then, be our rule, so that we may never go wrong between two things, one virtuous, the other useful. The upright man must never think of depriving another of anything, nor must he ever wish to increase his own advantage to the disadvantage of another. This rule the Apostle gives you, saying: “All things are lawful, but all things are not expedient; all things are lawful, but all things edify not. Let no man seek his own, but each one another's.” That is: Let no man seek his own advantage, but another's; let no man seek his own honour, but another's. Wherefore he says in another place: “Let each esteem other better than themselves, looking not each one to his own things, but to the things of others.”
14. And let no one seek his own favour or his own praise, but another's. This we can plainly see declared in the book of Proverbs, where the Holy Spirit says through Solomon: “My son, if you be wise, be wise for yourself and your neighbours; but if you turn out evil, thou alone shall bear it.” The wise man gives counsel to others, as the upright man does, and shares with him in wearing the form of either virtue.
Source: On the Duties of the Clergy (New Advent)