Objection 1. It would seem that the body is necessary for Happiness. For the perfection of virtue and grace presupposes the perfection of nature. But Happiness is the perfection of virtue and grace. Now the soul, without the body, has not the perfection of nature; since it is naturally a part of human nature, and every part is imperfect while separated from its whole. Therefore the soul cannot be happy without the body.
Objection 2. Further, Happiness is a perfect operation, as stated above (3, 2,5). But perfect operation follows perfect being: since nothing operates except in so far as it is an actual being. Since, therefore, the soul has not perfect being, while it is separated from the body, just as neither has a part, while separate from its whole; it seems that the soul cannot be happy without the body.
Objection 3. Further, Happiness is the perfection of man. But the soul, without the body, is not man. Therefore Happiness cannot be in the soul separated from the body.
Objection 4. Further, according to the Philosopher (Ethic. vii, 13) "the operation of bliss," in which operation happiness consists, is "not hindered." But the operation of the separate soul is hindered; because, as Augustine says (Gen. ad lit. xii, 35), the soul "has a natural desire to rule the body, the result of which is that it is held back, so to speak, from tending with all its might to the heavenward journey," i.e. to the vision of the Divine Essence. Therefore the soul cannot be happy without the body.
Objection 5. Further, Happiness is the sufficient good and lulls desire. But this cannot be said of the separated soul; for it yet desires to be united to the body, as Augustine says (Gen. ad lit. xii, 35). Therefore the soul is not happy while separated from the body.
Objection 6. Further, in Happiness man is equal to the angels. But the soul without the body is not equal to the angels, as Augustine says (Gen. ad lit. xii, 35). Therefore it is not happy.
On the contrary, It is written (Apocalypse 14:13): "Happy [Douay: 'blessed'] are the dead who die in the Lord."
I answer that, Happiness is twofold; the one is imperfect and is had in this life; the other is perfect, consisting in the vision of God. Now it is evident that the body is necessary for the happiness of this life. For the happiness of this life consists in an operation of the intellect, either speculative or practical. And the operation of the intellect in this life cannot be without a phantasm, which is only in a bodily organ, as was shown in the I, 84, 6,7. Consequently that happiness which can be had in this life, depends, in a way, on the body. But as to perfect Happiness, which consists in the vision of God, some have maintained that it is not possible to the soul separated from the body; and have said that the souls of saints, when separated from their bodies, do not attain to that Happiness until the Day of Judgment, when they will receive their bodies back again. And this is shown to be false, both by authority and by reason. By authority, since the Apostle says (2 Corinthians 5:6): "While we are in the body, we are absent from the Lord"; and he points out the reason of this absence, saying: "For we walk by faith and not by sight." Now from this it is clear that so long as we walk by faith and not by sight, bereft of the vision of the Divine Essence, we are not present to the Lord. But the souls of the saints, separated from their bodies, are in God's presence; wherefore the text continues: "But we are confident and have a good will to be absent... from the body, and to be present with the Lord." Whence it is evident that the souls of the saints, separated from their bodies, "walk by sight," seeing the Essence of God, wherein is true Happiness.
Again this is made clear by reason. For the intellect needs not the body, for its operation, save on account of the phantasms, wherein it looks on the intelligible truth, as stated in the I, 84, 7. Now it is evident that the Divine Essence cannot be seen by means of phantasms, as stated in the I, 12, 3. Wherefore, since man's perfect Happiness consists in the vision of the Divine Essence, it does not depend on the body. Consequently, without the body the soul can be happy.
We must, however, notice that something may belong to a thing's perfection in two ways. First, as constituting the essence thereof; thus the soul is necessary for man's perfection. Secondly, as necessary for its well-being: thus, beauty of body and keenness of perfection belong to man's perfection. Wherefore though the body does not belong in the first way to the perfection of human Happiness, yet it does in the second way. For since operation depends on a thing's nature, the more perfect is the soul in its nature, the more perfectly it has its proper operation, wherein its happiness consists. Hence, Augustine, after inquiring (Gen. ad lit. xii, 35) "whether that perfect Happiness can be ascribed to the souls of the dead separated from their bodies," answers "that they cannot see the Unchangeable Substance, as the blessed angels see It; either for some other more hidden reason, or because they have a natural desire to rule the body."
Reply to Objection 1. Happiness is the perfection of the soul on the part of the intellect, in respect of which the soul transcends the organs of the body; but not according as the soul is the natural form of the body. Wherefore the soul retains that natural perfection in respect of which happiness is due to it, though it does not retain that natural perfection in respect of which it is the form of the body.
Reply to Objection 2. The relation of the soul to being is not the same as that of other parts: for the being of the whole is not that of any individual part: wherefore, either the part ceases altogether to be, when the whole is destroyed, just as the parts of an animal, when the animal is destroyed; or, if they remain, they have another actual being, just as a part of a line has another being from that of the whole line. But the human soul retains the being of the composite after the destruction of the body: and this because the being of the form is the same as that of its matter, and this is the being of the composite. Now the soul subsists in its own being, as stated in the I, 75, 2. It follows, therefore, that after being separated from the body it has perfect being and that consequently it can have a perfect operation; although it has not the perfect specific nature.
Reply to Objection 3. Happiness belongs to man in respect of his intellect: and, therefore, since the intellect remains, it can have Happiness. Thus the teeth of an Ethiopian, in respect of which he is said to be white, can retain their whiteness, even after extraction.
Reply to Objection 4. One thing is hindered by another in two ways. First, by way of opposition; thus cold hinders the action of heat: and such a hindrance to operation is repugnant to Happiness. Secondly, by way of some kind of defect, because, to wit, that which is hindered has not all that is necessary to make it perfect in every way: and such a hindrance to operation is not incompatible with Happiness, but prevents it from being perfect in every way. And thus it is that separation from the body is said to hold the soul back from tending with all its might to the vision of the Divine Essence. For the soul desires to enjoy God in such a way that the enjoyment also may overflow into the body, as far as possible. And therefore, as long as it enjoys God, without the fellowship of the body, its appetite is at rest in that which it has, in such a way, that it would still wish the body to attain to its share.
Reply to Objection 5. The desire of the separated soul is entirely at rest, as regards the thing desired; since, to wit, it has that which suffices its appetite. But it is not wholly at rest, as regards the desirer, since it does not possess that good in every way that it would wish to possess it. Consequently, after the body has been resumed, Happiness increases not in intensity, but in extent.
Reply to Objection 6. The statement made (Gen. ad lit. xii, 35) to the effect that "the souls of the departed see not God as the angels do," is not to be understood as referring to inequality of quantity; because even now some souls of the Blessed are raised to the higher orders of the angels, thus seeing God more clearly than the lower angels. But it refers to inequality of proportion: because the angels, even the lowest, have every perfection of Happiness that they ever will have, whereas the separated souls of the saints have not.
Source: Summa Theologica (New Advent)