Objection 1. It would seem that happiness is not an operation. For the Apostle says (Romans 6:22): "You have your fruit unto sanctification, and the end, life everlasting." But life is not an operation, but the very being of living things. Therefore the last end, which is happiness, is not an operation.
Objection 2. Further, Boethius says (De Consol. iii) that happiness is "a state made perfect by the aggregate of all good things." But state does not indicate operation. Therefore happiness is not an operation.
Objection 3. Further, happiness signifies something existing in the happy one: since it is man's final perfection. But the meaning of operation does not imply anything existing in the operator, but rather something proceeding therefrom. Therefore happiness is not an operation.
Objection 4. Further, happiness remains in the happy one. Now operation does not remain, but passes. Therefore happiness is not an operation.
Objection 5. Further, to one man there is one happiness. But operations are many. Therefore happiness is not an operation.
Objection 6. Further, happiness is in the happy one uninterruptedly. But human operation is often interrupted; for instance, by sleep, or some other occupation, or by cessation. Therefore happiness is not an operation.
On the contrary, The Philosopher says (Ethic. i, 13) that "happiness is an operation according to perfect virtue."
I answer that, In so far as man's happiness is something created, existing in him, we must needs say that it is an operation. For happiness is man's supreme perfection. Now each thing is perfect in so far as it is actual; since potentiality without act is imperfect. Consequently happiness must consist in man's last act. But it is evident that operation is the last act of the operator, wherefore the Philosopher calls it "second act" (De Anima ii, 1): because that which has a form can be potentially operating, just as he who knows is potentially considering. And hence it is that in other things, too, each one is said to be "for its operation" (De Coel ii, 3). Therefore man's happiness must of necessity consist in an operation.
Reply to Objection 1. Life is taken in two senses. First for the very being of the living. And thus happiness is not life: since it has been shown (2, 05) that the being of a man, no matter in what it may consist, is not that man's happiness; for of God alone is it true that His Being is His Happiness. Secondly, life means the operation of the living, by which operation the principle of life is made actual: thus we speak of active and contemplative life, or of a life of pleasure. And in this sense eternal life is said to be the last end, as is clear from John 17:3: "This is eternal life, that they may know Thee, the only true God."
Reply to Objection 2. Boethius, in defining happiness, considered happiness in general: for considered thus it is the perfect common good; and he signified this by saying that happiness is "a state made perfect by the aggregate of all good things," thus implying that the state of a happy man consists in possessing the perfect good. But Aristotle expressed the very essence of happiness, showing by what man is established in this state, and that it is by some kind of operation. And so it is that he proves happiness to be "the perfect good" (Ethic. i, 7).
Reply to Objection 3. As stated in Metaph. ix, 7 action is twofold. One proceeds from the agent into outward matter, such as "to burn" and "to cut." And such an operation cannot be happiness: for such an operation is an action and a perfection, not of the agent, but rather of the patient, as is stated in the same passage. The other is an action that remains in the agent, such as to feel, to understand, and to will: and such an action is a perfection and an act of the agent. And such an operation can be happiness.
Reply to Objection 4. Since happiness signifies some final perfection; according as various things capable of happiness can attain to various degrees of perfection, so must there be various meanings applied to happiness. For in God there is happiness essentially; since His very Being is His operation, whereby He enjoys no other than Himself. In the happy angels, the final perfection is in respect of some operation, by which they are united to the Uncreated Good: and this operation of theirs is one only and everlasting. But in men, according to their present state of life, the final perfection is in respect of an operation whereby man is united to God: but this operation neither can be continual, nor, consequently, is it one only, because operation is multiplied by being discontinued. And for this reason in the present state of life, perfect happiness cannot be attained by man. Wherefore the Philosopher, in placing man's happiness in this life (Ethic. i, 10), says that it is imperfect, and after a long discussion, concludes: "We call men happy, but only as men." But God has promised us perfect happiness, when we shall be "as the angels... in heaven" (Matthew 22:30).
Consequently in regard to this perfect happiness, the objection fails: because in that state of happiness, man's mind will be united to God by one, continual, everlasting operation. But in the present life, in as far as we fall short of the unity and continuity of that operation so do we fall short of perfect happiness. Nevertheless it is a participation of happiness: and so much the greater, as the operation can be more continuous and more one. Consequently the active life, which is busy with many things, has less of happiness than the contemplative life, which is busied with one thing, i.e. the contemplation of truth. And if at any time man is not actually engaged in this operation, yet since he can always easily turn to it, and since he ordains the very cessation, by sleeping or occupying himself otherwise, to the aforesaid occupation, the latter seems, as it were, continuous. From these remarks the replies to Objections 5 and 6 are evident.
Source: Summa Theologica (New Advent)