The different ideas of philosophers on the subject of happiness. He proves, first, from the Gospel that it rests on the knowledge of God and the pursuit of good works; next, that it may not be thought that this idea was adopted from the philosophers, he adds proofs from the witness of the prophets.
4. The philosophers have made a happy life to depend, either (as Hieronymus) on freedom from pain, or (as Herillus) on knowledge. For Herillus, hearing knowledge very highly praised by Aristotle and Theophrastus, made it alone to be the chief good, when they really praised it as a good thing, not as the only good; others, as Epicurus, have called pleasure such; others, as Callipho, and after him Diodorus, understood it in such a way as to make a virtuous life go in union, the one with pleasure, the other with freedom from pain, since a happy life could not exist without it. Zeno, the Stoic, thought the highest and only good existed in a virtuous life. But Aristotle and Theophrastus and the other Peripatetics maintained that a happy life consisted in virtue, that is, in a virtuous life, but that its happiness was made complete by the advantages of the body and other external good things.
5. But the sacred Scriptures say that eternal life rests on a knowledge of divine things and on the fruit of good works. The Gospel bears witness to both these statements. For the Lord Jesus spoke thus of knowledge: “This is eternal life, to know You, the only true God, and Jesus Christ Whom You have sent.” About works He gives this answer: “Every one that has forsaken house, or brethren, or sisters, or father, or mother, or wife, or children, or lands, for My Name's sake, shall receive an hundred-fold, and shall inherit everlasting life.”
6. Let no one think that this was but lately said, and that it was spoken of by the philosophers before it was mentioned in the Gospel. For the philosophers, that is to say, Aristotle and Theophrastus, as also Zeno and Hieronymus, certainly lived before the time of the Gospel; but they came after the prophets. Let them rather think how long before even the names of the philosophers were heard of, both of these seem to have found open expression through the mouth of the holy David; for it is written: “Blessed is the man whom You instruct, O Lord, and teach him out of Your law.” We find elsewhere also: “Blessed is the man that fears the Lord, he will rejoice greatly in His commandments.” We have proved our point as regards knowledge, the reward for which the prophet states to be the fruit of eternity, adding that in the house of the man that fears the Lord, or is instructed in His law and rejoices greatly in the divine commandments, “is glory and riches; and his justice abides for ever and ever.” He has further also in the same psalm stated of good works, that they gain for an upright man the gift of eternal life. He speaks thus: “Blessed is the man that shows pity and lends, he will guide his affairs with discretion, surely he shall not be moved for ever, the righteous shall be in everlasting remembrance.” And further: “He has dispersed, he has given to the poor, his justice endures for ever.”
7. Faith, then, has [the promise of] eternal life, for it is a good foundation. Good works, too, have the same, for an upright man is tested by his words and acts. For if a man is always busy talking and yet is slow to act, he shows by his acts how worthless his knowledge is: besides it is much worse to know what one ought to do, and yet not to do what one has learned should be done. On the other hand, to be active in good works and unfaithful at heart is as idle as though one wanted to raise a beautiful and lofty dome upon a bad foundation. The higher one builds, the greater is the fall; for without the protection of faith good works cannot stand. A treacherous anchorage in a harbour perforates a ship, and a sandy bottom quickly gives way and cannot bear the weight of the building placed upon it. There then will be found the fullness of reward, where the virtues are perfect, and where there is a reasonable agreement between words and acts.
Source: On the Duties of the Clergy (New Advent)