Objection 1. It would seem that it will not be identically the same man that shall rise again. For according to the Philosopher (De Gener. ii): "Whatsoever things are changed in their corruptible substance are not repeated identically." Now such is man's substance in his present state. Therefore after the change wrought by death the self-same man cannot be repeated.
Objection 2. Further, where there is a distinction of human nature there is not the same identical man: wherefore Socrates and Plato are two men and not one man, since each has his own distinct human nature. Now the human nature of one who rises again is distinct from that which he has now. Therefore he is not the same identical man. The minor can be proved in two ways. First, because human nature which is the form of the whole is not both form and substance as the soul is, but is a form only. Now such like forms pass away into complete nonentity, and consequently they cannot be restored. Secondly, because human nature results from union of parts. Now the same identical union as that which was heretofore cannot be resumed, because repetition is opposed to identity, since repetition implies number, whereas identity implies unity, and these are incompatible with one another. But resurrection is a repeated union: therefore the union is not the same, and consequently there is not the same human nature nor the same man.
Objection 3. Further, one same man is not several animals: wherefore if it is not the same animal it is not the same identical man. Now where sense is not the same, there is not the same animal, since animal is defined from the primary sense, namely touch. But sense, as it does not remain in the separated soul (as some maintain), cannot be resumed in identity. Therefore the man who rises again will not be the same identical animal, and consequently he will not be the same man.
Objection 4. Further, the matter of a statue ranks higher in the statue than the matter of a man does in man: because artificial things belong to the genus of substance by reason of their matter, but natural things by reason of their form, as appears from the Philosopher (Phys. ii, 1), and again from the Commentator (De Anima ii). But if a statue is remade from the same brass, it will not be the same identically. Therefore much less will it be identically the same man if he be reformed from the same ashes.
On the contrary, It is written (Job 19:27): "Whom I myself shall see... and not another," and he is speaking of the vision after the resurrection. Therefore the same identical man will rise again.
Further, Augustine says (De Trin. viii, 5) that "to rise again is naught else but to live again." Now unless the same identical man that died return to life, he would not be said to live again. Therefore he would not rise again, which is contrary to faith.
I answer that, The necessity of holding the resurrection arises from this--that man may obtain the last end for which he was made; for this cannot be accomplished in this life, nor in the life of the separated soul, as stated above (75, 1,2): otherwise man would have been made in vain, if he were unable to obtain the end for which he was made. And since it behooves the end to be obtained by the selfsame thing that was made for that end, lest it appear to be made without purpose, it is necessary for the selfsame man to rise again; and this is effected by the selfsame soul being united to the selfsame body. For otherwise there would be no resurrection properly speaking, if the same man were not reformed. Hence to maintain that he who rises again is not the selfsame man is heretical, since it is contrary to the truth of Scripture which proclaims the resurrection.
Reply to Objection 1. The Philosopher is speaking of repetition by movement or natural change. For he shows the difference between the recurrence that occurs in generation and corruption and that which is observed in the movement of the heavens. Because the selfsame heaven by local movement returns to the beginning of its movement, since it has a moved incorruptible substance. On the other hand, things subject to generation and corruption return by generation to specific but not numerical identity, because from man blood is engendered, from blood seed, and so on until a man is begotten, not the selfsame man, but the man specifically. In like manner from fire comes air, from air water, from water earth, whence fire is produced, not the selfsame fire, but the same in species. Hence it is clear that the argument, so far as the meaning of the Philosopher is concerned, is not to the point.
We may also reply that the form of other things subject to generation and corruption is not subsistent of itself, so as to be able to remain after the corruption of the composite, as it is with the rational soul. For the soul, even after separation from the body, retains the being which accrues to it when in the body, and the body is made to share that being by the resurrection, since the being of the body and the being of the soul in the body are not distinct from one another, otherwise the union of soul and body would be accidental. Consequently there has been no interruption in the substantial being of man, as would make it impossible for the self-same man to return on account of an interruption in his being, as is the case with other things that are corrupted, the being of which is interrupted altogether, since their form remains not, and their matter remains under another being.
Nevertheless neither does the self-same man recur by natural generation, because the body of the man begotten is not composed of the whole body of his begetter: hence his body is numerically distinct, and consequently his soul and the whole man.
Reply to Objection 2. There are two opinions about humanity and about any form of a whole. For some say that the form of the whole and the form of the part are really one and the same: but that it is called the form of the part inasmuch as it perfects the matter, and the form of the whole inasmuch as the whole specific nature results therefrom. According to this opinion humanity is really nothing else than the rational soul: and so, since the selfsame rational soul is resumed, there will be the same identical humanity, which will remain even after death, albeit not under the aspect of humanity, because the composite does not derive the specific nature from a separated humanity.
The other opinion, which seems nearer the truth, is Avicenna's, according to whom the form of the whole is not the form of a part only, nor some other form besides the form of the part, but is the whole resulting from the composition of form and matter, embracing both within itself. This form of the whole is called the essence or quiddity. Since then at the resurrection there will be the selfsame body, and the selfsame rational soul, there will be, of necessity, the same humanity.
The first argument proving that there will be a distinction of humanity was based on the supposition that humanity is some distinct form supervening form and matter; which is false.
The second reason does not disprove the identity of humanity, because union implies action or passion, and though there be a different union, this cannot prevent the identity of humanity, because the action and passion from which humanity resulted are not of the essence of humanity, wherefore a distinction on their part does not involve a distinction of humanity: for it is clear that generation and resurrection are not the self-same movement. Yet the identity of the rising man with the begotten man is not hindered for this reason: and in like manner neither is the identity of humanity prevented if we take union for the relation itself: because this relation is not essential to but concomitant with humanity, since humanity is not one of those forms that are composition or order (Phys. ii, 1), as are the forms of things produced by art, so that if there be another distinct composition there is another distinct form of a house.
Reply to Objection 3. This argument affords a very good proof against those who held a distinction between the sensitive and rational souls in man: because in that case the sensitive soul in man would not be incorruptible, as neither is it in other animals; and consequently in the resurrection there would not be the same sensitive soul, and consequently neither the same animal nor the same man.
But if we assert that in man the same soul is by its substance both rational and sensitive, we shall encounter no difficulty in this question, because animal is defined from sense, i.e. the sensitive soul as from its essential form: whereas from sense, i.e. the sensitive power, we know its definition as from an accidental form "that contributes more than another to our knowledge of the quiddity" (De Anima i, 1). Accordingly after death there remains the sensitive soul, even as the rational soul, according to its substance: whereas the sensitive powers, according to some, do not remain. And since these powers are accidental properties, diversity on their part cannot prevent the identity of the whole animal, not even of the animal's parts: nor are powers to be called perfections or acts of organs unless as principles of action, as heat in fire.
Reply to Objection 4. A statue may be considered in two ways, either as a particular substance, or as something artificial. And since it is placed in the genus of substance by reason of its matter, it follows that if we consider it as a particular substance, it is the selfsame statue that is remade from the same matter. On the other hand, it is placed in the genus of artificial things inasmuch as it has an accidental form which, if the statue be destroyed, passes away also. Consequently it does not return identically the same, nor can the statue be identically the same. But man's form, namely the soul, remains after the body has perished: wherefore the comparison fails.
Source: Summa Theologica (New Advent)