Objection 1. It would seem that the soul will not be reunited to the same identical body at the resurrection, for "thou sowest not the body that shall be, but bare grain" (1 Corinthians 15:37). Now the Apostle is there comparing death to sowing and resurrection to fructifying. Therefore the same body that is laid aside in death is not resumed at the resurrection.
Objection 2. Further, to every form some matter is adapted according to its condition, and likewise to every agent some instrument. Now the body is compared to the soul as matter to form, and as instrument to agent. Since then at the resurrection the soul will not be of the same condition as now (for it will be either entirely borne away to the heavenly life to which it adhered while living in the world, or will be cast down into the life of the brutes if it lived as a brute in this world) it would seem that it will not resume the same body, but either a heavenly or a brutish body.
Objection 3. Further, after death, as stated above (Question 78, Article 3), the human body is dissolved into the elements. Now these elemental parts into which the human body has been dissolved do not agree with the human body dissolved into them, except in primary matter, even as any other elemental parts agree with that same body. But if the body were to be formed from those other elemental parts, it would not be described as identically the same. Therefore neither will it be the self-same body if it be restored from these parts.
Objection 4. Further, there cannot be numerical identity where there is numerical distinction of essential parts. Now the form of the mixed body, which form is an essential part of the human body, as being its form, cannot be resumed in numerical identity. Therefore the body will not be identically the same. The minor is proved thus: That which passes away into complete nonentity cannot be resumed in identity. This is clear from the fact that there cannot be identity where there is distinction of existence: and existence, which is the act of a being, is differentiated by being interrupted, as is any interrupted act. Now the form of a mixed body passes away into complete nonentity by death, since it is a bodily form, and so also do the contrary qualities from which the mixture results. Therefore the form of a mixed body does not return in identity.
On the contrary, It is written (Job 19:26): "In my flesh I shall see God my Saviour [Vulgate: 'my God']," where he is speaking of the vision after the resurrection, as appears from the preceding words: "In the last day I shall rise out of the earth." Therefore the selfsame body will rise again.
Further, the Damascene says (De Fide Orth. iv, 27): "Resurrection is the second rising of that which has fallen." But the body which we have now fell by death. Therefore it will rise again the same identically.
I answer that, On this point the philosophers erred and certain modern heretics err. For some of the philosophers allowed that souls separated from bodies are reunited to bodies, yet they erred in this in two ways. First, as to the mode of reunion, for some held the separated soul to be naturally reunited to a body by the way of generation. Secondly, as to the body to which it was reunited, for they held that this second union was not with the selfsame body that was laid aside in death, but with another, sometimes of the same, sometimes of a different species. Of a different species when the soul while existing in the body had led a life contrary to the ordering of reason: wherefore it passed after death from the body of a man into the body of some other animal to whose manner of living it had conformed in this life, for instance into the body of a dog on account of lust, into the body of a lion on account of robbery and violence, and so forth--and into a body of the same species when the soul has led a good life in the body, and having after death experienced some happiness, after some centuries began to wish to return to the body; and thus it was reunited to a human body.
This opinion arises from two false sources. The first of these is that they said that the soul is not united to the body essentially as form to matter, but only accidentally, as mover to the thing moved, [Cf. I, 76, 1] or as a man to his clothes. Hence it was possible for them to maintain that the soul pre-existed before being infused into the body begotten of natural generation, as also that it is united to various bodies. The second is that they held intellect not to differ from sense except accidentally, so that man would be said to surpass other animals in intelligence, because the sensitive power is more acute in him on account of the excellence of his bodily complexion; and hence it was possible for them to assert that man's soul passes into the soul of a brute animal, especially when the human soul has been habituated to brutish actions. But these two sources are refuted by the Philosopher (De Anima ii, 1), and in consequence of these being refuted, it is clear that the above opinion is false.
In like manner the errors of certain heretics are refuted. Some of them fell into the aforesaid opinions of the philosophers: while others held that souls are reunited to heavenly bodies, or again to bodies subtle as the wind, as Gregory relates of a certain Bishop of Constantinople, in his exposition of Job 19:26, "In my flesh I shall see my God," etc. Moreover these same errors of heretics may be refuted by the fact that they are prejudicial to the truth of resurrection as witnessed to by Holy Writ. For we cannot call it resurrection unless the soul return to the same body, since resurrection is a second rising, and the same thing rises that falls: wherefore resurrection regards the body which after death falls rather than the soul which after death lives. And consequently if it be not the same body which the soul resumes, it will not be a resurrection, but rather the assuming of a new body.
Reply to Objection 1. A comparison does not apply to every particular, but to some. For in the sowing of grain, the grain sown and the grain that is born thereof are neither identical, nor of the same condition, since it was first sown without a husk, yet is born with one: and the body will rise again identically the same, but of a different condition, since it was mortal and will rise in immortality.
Reply to Objection 2. The soul rising again and the soul living in this world differ, not in essence but in respect of glory and misery, which is an accidental difference. Hence it follows that the body in rising again differs, not in identity, but in condition, so that a difference of bodies corresponds proportionally to the difference of souls.
Reply to Objection 3. That which is understood as though it were in matter before its form remains in matter after corruption, because when that which comes afterwards is removed that which came before may yet remain. Now, as the Commentator observes on the First Book of Physics and in De Substantia Orbis, in the matter of things subject to generation and corruption, we must presuppose undeterminate dimensions, by reason of which matter is divisible, so as to be able to receive various forms in its various parts. Wherefore after the separation of the substantial form from matter, these dimensions still remain the same: and consequently the matter existing under those dimensions, whatever form it receive, is more identified with that which was generated from it, than any other part of matter existing under any form whatever. Thus the matter that will be brought back to restore the human body will be the same as that body's previous matter.
Reply to Objection 4. Even as a simple quality is not the substantial form of an element, but its proper accident, and the disposition whereby its matter is rendered proper to such a form; so the form of a mixed body, which form is a quality resulting from simple qualities reduced to a mean, is not the substantial form of the mixed body, but its proper accident, and the disposition whereby the matter is in need of the form. Now the human body has no substantial form besides this form of the mixed body, except the rational soul, for if it had any previous substantial form, this would give it substantial being, and would establish it in the genus of substance: so that the soul would be united to a body already established in the genus of substance, and thus the soul would be compared to the body as artificial forms are to their matter, in respect of their being established in the genus of substance by their matter. Hence the union of the soul to the body would be accidental, which is the error of the ancient philosophers refuted by the Philosopher (De Anima ii, 2 [Cf. I, 76, 1]. It would also follow that the human body and each of its parts would not retain their former names in the same sense, which is contrary to the teaching of the Philosopher (De Anima ii, 1). Therefore since the rational soul remains, no substantial form of the human body falls away into complete nonentity. And the variation of accidental forms does not make a difference of identity. Therefore the selfsame body will rise again, since the selfsame matter is resumed as stated in a previous reply (ad 2).
Source: Summa Theologica (New Advent)