A duty is to be chosen from what is virtuous, and from what is useful, and also from the comparison of the two, one with the other; but nothing is recognized by Christians as virtuous or useful which is not helpful to the future life. This treatise on duty, therefore, will not be superfluous.
27. The philosophers considered that duties were derived from what is virtuous and what is useful, and that from these two one should choose the better. Then, they say, it may happen that two virtuous or two useful things will clash together, and the question is, which is the more virtuous, and which the more useful? First, therefore, “duty” is divided into three sections: what is virtuous, what is useful, and what is the better of two. Then, again, these three are divided into five classes; that is, two that are virtuous, two that are useful, and, lastly, the right judgment as to the choice between them. The first they say has to do with the moral dignity and integrity of life; the second with the con veniences of life, with wealth, resources, opportunities; while a right judgment must underlie the choice of any of them. This is what the philosophers say.
28. But we measure nothing at all but that which is fitting and virtuous, and that by the rule of things future rather than of things present; and we state nothing to be useful but what will help us to the blessing of eternal life; certainly not that which will help us enjoy merely the present time. Nor do we recognize any advantages in opportunities and in the wealth of earthly goods, but consider them as disadvantages if not put aside, and to be looked on as a burden, when we have them, rather than as a loss when expended.
29. This work of ours, therefore, is not superfluous, seeing that we and they regard duty in quite different ways. They reckon the advantages of this life among the good things, we reckon them among the evil things; for he who receives good things here, as the rich man in the parable, is tormented there; and Lazarus, who endured evil things here, there found comfort. Lastly, those who do not read their writings may read ours if they will— if, that is, they do not require great adornment of language or a skilfully-treated subject, but are satisfied with the simple charm of the subject itself.
Source: On the Duties of the Clergy (New Advent)