St. Thomas aims to prove that there is such a thing as “last end” for humans. He correctly identifies it as happiness. Further, he believes that it is specifically “the sole contemplation of God seen in His essence” that grants perfect pleasure. It’s what puts the mind at ease and rest, what satisfies all desire: the vision and secure possession of all truth, even if God remains infinite and beyond full comprehension for any finite creature. Thus, “if man’s happiness is an operation, it must needs be man’s highest operation. Now man’s highest operation is that of his highest power in respect of its highest object: and his highest power is the intellect, whose highest object is the Divine Good, which is the object, not of the practical but of the speculative intellect. … Therefore the last and perfect happiness, which we await in the life to come, consists entirely in contemplation.” (ST, II-I, 3, 5)
I have two objections to this.
First is that there is no such thing as “perfect happiness” even in contemplation. The object of vision, God, obviously cannot improve, but the understanding of a blessed soul or angel can, with time. There has to be continuous learning of new things, new insights, including even by the angels. The idea of a permanent changeless operation is repugnant to the nature of all rational creatures as potentially infinite.
Second is that the human body is not a disposable consumer good, as though “the very body is for the soul, as matter for its form, and the instruments for the man that puts them into motion, that by their means he may do his work.” (II-I, 2, 5) The body is not like a shovel or granola bar but an essential aspect of humanity; and this is precisely how we differ from the angels. The embodied pleasures for humans in paradise will be equal in intensity and fun to the disembodied pleasure of contemplating God in heaven.
For example, St. Thomas is wrong in thinking that the “the fellowship of friends” is not necessary for happiness: “Perfection of charity is essential to happiness, as to the love of God, but not as to the love of our neighbor. Wherefore if there were but one soul enjoying God, it would be happy, though having no neighbor to love.” (II-I, 4, 8, reply 3) He seems to contradict himself in (I, 106): even regarding contemplation, he argues that superior angels enlighten inferior ones in the celestial hierarchy and always will. Much more then do angels and greater saints enlighten the lesser souls in the state of glory.
Even if contemplation of God is a somewhat solitary activity, in paradise a solitary human would be rather miserable. Humans are highly social creatures, complementing each other in a vast variety of ways. To deprive us from the communion of saints is to harm us immensely and unjustly. Thus, Jesus’ words to the crucified thief were not, “Today you will be with the Father in heaven,” but “Today you will be with me in paradise.” (Lk 23:43) It seems therefore that in heaven the Holy Spirit will unite each man with the Father; and in paradise, He will unite us with each other and with the Son.
The whole reason for the enormous sacrifices God has made through the fall of angels and men and the death of His Son and massive human suffering on earth has been to unite the universe into a one thing through different types of love or charity. To turn around and blithely proclaim that the communion between humans, between men and nature, and so forth, as described earlier, is all of a sudden actually irrelevant or at most an afterthought is a major mistake. We can excuse St. Thomas only because in his times, neither economic science nor laissez-faire capitalism existed, and the idea of everlasting progress in man’s active life was completely unknown and unentertained.
St. Thomas’ proof that one’s contemplative life is perfected by the vision of God in heaven is adequate: all intellectual wonder is satisfied by beholding the source and archetype of all things. That in addition to heaven, there is also paradise in which one’s active life is perfected requires a different proof. Or rather, the idea of paradise can be developed a priori, but whether paradise actually exists is an empirical problem which can be resolved by considering the Incarnation of the Son. We know that the Incarnation occurred, that Jesus is God, that He was born so that His love for us could be tested by our ultimate injustice toward Him personally — by following the evidence. Since we know that Jesus united human nature, soul and body both, to the divinity, we can deduce that the happiness in the embodied active life can be one of our rewards in the hereafter.
The passage, “What eye has not seen, and ear has not heard, and what has not entered the human heart, what God has prepared for those who love him,” (1 Cor 2:9) is best interpreted as the promise of future enjoyments in paradise, since, contrary to it, we do know exactly what God has prepared for man’s contemplation in heaven, and that is Himself.
Now we must make a distinction between our ability to conceive what will make a man happy and whether this happiness is reachable. Now regarding the contemplative life, St. Thomas argues in (I, 12, 4) that no created intellect by its natural powers can see the divine essence. And regarding the active life, reason tells us that humans not only fail to enjoy everlasting improvement and perpetual novel pleasures in paradise, but that they fail to secure life itself, since everyone dies including often in terrible pain. Therefore, that both kinds of happiness are attainable by man is a deliverance not of reason but of faith.
And with that, we have it that union with God is our last end for our both contemplative and active lives.