Objection 1. It would seem necessary for the ashes of the human body to return, by the resurrection, to the same parts that were dissolved into them. For, according to the Philosopher, "as the whole soul is to the whole body, so is a part of the soul to a part of the body, as sight to the pupil" (De Anima ii, 1). Now it is necessary that after the resurrection the body be resumed by the same soul. Therefore it is also necessary for the same parts of the body to return to the same limbs, in which they were perfected by the same parts of the soul.
Objection 2. Further, difference of matter causes difference of identity. But if the ashes return not to the same parts, each part will not be remade from the same matter of which it consisted before. Therefore they will not be the same identically. Now if the parts are different the whole will also be different, since parts are to the whole as matter is to form (Phys. ii, 3). Therefore it will not be the self-same man; which is contrary to the truth of the resurrection.
Objection 3. Further, the resurrection is directed to the end that man may receive the meed of his works. Now different parts of the body are employed in different works, whether of merit or of demerit. Therefore at the resurrection each part must needs return to its former state that it may be rewarded in due measure.
On the contrary, Artificial things are more dependent on their matter than natural things. Now in artificial things, in order that the same artificial thing be remade, from the same matter, there is no need for the parts to be brought back to the same position. Neither therefore is it necessary in man.
Further, change of an accident does not cause a change of identity. Now the situation of parts is an accident. Therefore its change in a man does not cause a change of identity.
I answer that, In this question it makes a difference whether we ask what can be done without prejudice to identity, and what will be done for the sake of congruity. As regards the first it must be observed that in man we may speak of parts in two ways: first as of the various parts of a homogeneous whole, for instance the various parts of flesh, or the various parts of bone; secondly, as of various parts of various species of a heterogeneous whole, for instance bone and flesh. Accordingly if it be said that one part of matter will return to another part of the same species, this causes no change except in the position of the parts: and change of position of parts does not change the species in homogeneous wholes: and so if the matter of one part return to another part, this is nowise prejudicial to the identity of the whole. Thus is it in the example given in the text (Sent. iv, D, 44), because a statue, after being remade, is identically the same, not as to its form, but as to its matter, in respect of which it is a particular substance, and in this way a statue is homogeneous, although it is not according to its artificial form. But if it be said that the matter of one part returns to another part of another species, it follows of necessity that there is a change not only in the position of parts, but also in their identity: yet so that the whole matter, or something belonging to the truth of human nature in one is transferred to another. but not if what was superfluous in one part is transferred to another. Now the identity of parts being taken away, the identity of the whole is removed, if we speak of essential parts, but not if we speak of accidental parts, such as hair and nails, to which apparently Augustine refers (De Civ. Dei xxii). It is thus clear how the transference of matter from one part of another destroys the identity, and how it does not.
But speaking of the congruity, it is more probable that even the parts will retain their position at the resurrection, especially as regards the essential and organic parts, although perhaps not as regards the accidental parts, such as nails and hair.
Reply to Objection 1. This argument considers organic or heterogeneous parts, but no homogeneous or like parts.
Reply to Objection 2. A change in the position of the parts of matter does not cause a change of identity, although difference of matter does.
Reply to Objection 3. Operation, properly speaking, is not ascribed to the part but to the whole, wherefore the reward is due, not to the part but to the whole.
Source: Summa Theologica (New Advent)