Objection 1. It would seem that there is not to be a resurrection of the body: for it is written (Job 14:12): "Man, when he is fallen asleep, shall not rise again till the heavens be broken." But the heavens shall never be broken, since the earth, to which seemingly this is still less applicable, "standeth for ever" (Ecclesiastes 1:4). Therefore the man that is dead shall never rise again.
Objection 2. Further, Our Lord proves the resurrection by quoting the words: "I am the God of Abraham, and the God of Isaac, and the God of Jacob. He is not the God of the dead but of the living" (Matthew 22:32; Exodus 3:6). But it is clear that when those words were uttered, Abraham, Isaac, and Jacob lived not in body, but only in the soul. Therefore there will be no resurrection of bodies but only of souls.
Objection 3. Further, the Apostle (1 Corinthians 15) seemingly proves the resurrection from the reward for labors endured by the saints in this life. For if they trusted in this life alone, they would be the most unhappy of all men. Now there can be sufficient reward for labor in the soul alone: since it is not necessary for the instrument to be repaid together with the worker, and the body is the soul's instrument. Wherefore even in purgatory, where souls will be punished for what they did in the body, the soul is punished without the body. Therefore there is no need to hold a resurrection of the body, but it is enough to hold a resurrection of souls, which consists in their being taken from the death of sin and unhappiness to the life of grace and glory.
Objection 4. Further, the last state of a thing is the most perfect, since thereby it attains its end. Now the most perfect state of the soul is to be separated from the body, since in that state it is more conformed to God and the angels, and is more pure, as being separated from any extraneous nature. Therefore separation from the body is its final state, and consequently it returns not from this state to the body, as neither does a man end in becoming a boy.
Objection 5. Further, bodily death is the punishment inflicted on man for his own transgression, as appears from Genesis 2, even as spiritual death, which is the separation of the soul from God, is inflicted on man for mortal sin. Now man never returns to life from spiritual death after receiving the sentence of his damnation. Therefore neither will there be any return from bodily death to bodily life, and so there will be no resurrection.
On the contrary, It is written (Job 19:25-26): "I know that my Redeemer liveth, and in the last day I shall rise out of the earth, and I shall be clothed again with my skin," etc. Therefore there will be a resurrection of the body.
Further, the gift of Christ is greater than the sin of Adam, as appears from Romans 5:15. Now death was brought in by sin, for if sin had not been, there had been no death. Therefore by the gift of Christ man will be restored from death to life.
Further, the members should be conformed to the head. Now our Head lives and will live eternally in body and soul, since "Christ rising again from the dead dieth now no more" (Romans 6:8). Therefore men who are His members will live in body and soul; and consequently there must needs be a resurrection of the body.
I answer that, According to the various opinions about man's last end there have been various opinions holding or denying the resurrection. For man's last end which all men desire naturally is happiness. Some have held that man is able to attain this end in this life: wherefore they had no need to admit another life after this, wherein man would be able to attain to his perfection: and so they denied the resurrection.
This opinion is confuted with sufficient probability by the changeableness of fortune, the weakness of the human body, the imperfection and instability of knowledge and virtue, all of which are hindrances to the perfection of happiness, as Augustine argues at the end of De Civ. Dei (xxii, 22).
Hence others maintained that after this there is another life wherein, after death, man lives according to the soul only, and they held that such a life sufficed to satisfy the natural desire to obtain happiness: wherefore Porphyrius said as Augustine states (De Civ. De. xxii, 26): "The soul, to be happy, must avoid all bodies": and consequently these did not hold the resurrection.
This opinion was based by various people on various false foundations. For certain heretics asserted that all bodily things are from the evil principle, but that spiritual things are from the good principle: and from this it follows that the soul cannot reach the height of its perfection unless it be separated from the body, since the latter withdraws it from its principle, the participation of which makes it happy. Hence all those heretical sects that hold corporeal things to have been created or fashioned by the devil deny the resurrection of the body. The falsehood of this principle has been shown at the beginning of the Second Book (Sent. ii, D, 4, qu. 1, 3; * [Cf. I, 49, 3]).
Others said that the entire nature of man is seated in the soul, so that the soul makes use of the body as an instrument, or as a sailor uses his ship: wherefore according to this opinion, it follows that if happiness is attained by the soul alone, man would not be balked in his natural desire for happiness, and so there is no need to hold the resurrection. But the Philosopher sufficiently destroys this foundation (De Anima ii, 2), where he shows that the soul is united to the body as form to matter. Hence it is clear that if man cannot be happy in this life, we must of necessity hold the resurrection.
Reply to Objection 1. The heavens will never be broken as to their substance, but as to the effect of their power whereby their movement is the cause of generation and corruption of lower things: for this reason the Apostle says (1 Corinthians 7:31): "The fashion of this world passeth away."
Reply to Objection 2. Abraham's soul, properly speaking, is not Abraham himself, but a part of him (and the same as regards the others). Hence life in Abraham's soul does not suffice to make Abraham a living being, or to make the God of Abraham the God of a living man. But there needs to be life in the whole composite, i.e. the soul and body: and although this life were not actually when these words were uttered, it was in each part as ordained to the resurrection. Wherefore our Lord proves the resurrection with the greatest subtlety and efficacy.
Reply to Objection 3. The soul is compared to the body, not only as a worker to the instrument with which he works, but also as form to matter: wherefore the work belongs to the composite and not to the soul alone, as the Philosopher shows (De Anima i, 4). And since to the worker is due the reward of the work, it behooves man himself, who is composed of soul and body, to receive the reward of his work. Now as venial offenses are called sins as being dispositions to sin, and not as having simply and perfectly the character of sin, so the punishment which is awarded to them in purgatory is not a retribution simply, but rather a cleansing, which is wrought separately in the body, by death and by its being reduced to ashes, and in the soul by the fire of purgatory.
Reply to Objection 4. Other things being equal, the state of the soul in the body is more perfect than outside the body, because it is a part of the whole composite; and every integral part is material in comparison to the whole: and though it were conformed to God in one respect, it is not simply. Because, strictly speaking, a thing is more conformed to God when it has all that the condition of its nature requires, since then most of all it imitates the Divine perfection. Hence the heart of an animal is more conformed to an immovable God when it is in movement than when it is at rest, because the perfection of the heart is in its movement, and its rest is its undoing.
Reply to Objection 5. Bodily death was brought about by Adam's sin which was blotted out by Christ's death: hence its punishment lasts not for ever. But mortal sin which causes everlasting death through impenitence will not be expiated hereafter. Hence that death will be everlasting.
Source: Summa Theologica (New Advent)