10 You ask, perchance, What is this happy life? On this question the talents and leisure of many philosophers have been wasted, who, nevertheless, failed in their researches after it just in proportion as they failed to honour Him from whom it proceeds, and were unthankful to Him. In the first place, then, consider whether we should accept the opinion of those philosophers who pronounce that man happy who lives according to his own will. Far be it, surely, from us to believe this; for what if a man's will inclines him to live in wickedness? Is he not proved to be a miserable man in proportion to the facility with which his depraved will is carried out? Even philosophers who were strangers to the worship of God have rejected this sentiment with deserved abhorrence. One of them, a man of the greatest eloquence, says: “Behold, however, others, not philosophers indeed, but men of ready power in disputation, who affirm that all men are happy who live according to their own will. But this is certainly untrue, for to wish that which is unbecoming is itself a most miserable thing; nor is it so miserable a thing to fail in obtaining what you wish as to wish to obtain what you ought not to desire.” What is your opinion? Are not these words, by whomsoever they are spoken, derived from the Truth itself? We may therefore here say what the apostle said of a certain Cretan poet whose sentiment had pleased him: “This witness is true.”
11. He, therefore, is truly happy who has all that he wishes to have, and wishes to have nothing which he ought not to wish. This being understood, let us now observe what things men may without impropriety wish to have. One desires marriage; another, having become a widower, chooses thereafter to live a life of continence; a third chooses to practise continence though he is married. And although of these three conditions one may be found better than another, we cannot say that any one of the three persons is wishing what he ought not: the same is true of the desire for children as the fruit of marriage, and for life and health to be enjoyed by the children who have been received—of which desires the latter is one with which widows remaining unmarried are for the most part occupied; for although, refusing a second marriage, they do not now wish to have children, they wish that the children that they have may live in health. From all such care those who preserve their virginity intact are free. Nevertheless, all have some dear to them whose temporal welfare they do without impropriety desire. But when men have obtained this health for themselves, and for those whom they love, are we at liberty to say that they are now happy? They have, it is true, something which it is quite becoming to desire; but if they have not other things which are greater, better, and more full both of utility and beauty, they are still far short of possessing a happy life.
Source: Letters (New Advent)