I answer that, "Everything is related to truth in the same way as to being" (Metaph. ii), because a thing is true when it is as it appears to him who actually knows it. For this reason Avicenna (Metaph. ii) says that "the truth of anything is a property of the being immutably attached thereto." Accordingly a thing is said to belong to the truth of human nature, because it belongs properly to the being of human nature, and this is what shares the form of human nature, just as true gold is what has the true form of gold whence gold derives its proper being. In order therefore to see what it is that belongs to the truth of human nature, we must observe that there have been three opinions on the question. For some have maintained that nothing begins anew to belong to the truth of human nature and that whatever belongs to the truth of human nature, all of it belonged to the truth of human nature when this was created; and that this multiplies by itself, so that it is possible for the seed whereof the child is begotten to be detached therefrom by the begetter, and that again the detached part multiplies in the child, so that he reaches perfect quantity by growth, and so on, and that thus was the whole human race multiplied. Wherefore according to this opinion, whatever is produced by nourishment. although it seem to have the appearance of flesh and blood, does not belong to the truth of human nature.
Others held that something new is added to the truth of human nature by the natural transformation of the food into the human body, if we consider the truth of human nature in the species to the preservation of which the act of the generative power is directed: but that if we consider the truth of human nature in the individual, to the preservation and perfection of which the act of the nutritive power is directed, that which is added by food belongs to the truth of the human nature of the individual, not primarily but secondarily. For they assert that the truth of human nature, first and foremost, consists in the radical humor, that namely which is begotten of the seed of which the human race was originally fashioned: and that what is changed from food into true flesh and blood does not belong principally to the truth of human nature in this particular individual, but secondarily: and that nevertheless this can belong principally to the truth of human nature in another individual who is begotten of the seed of the former. For they assert that seed is the surplus from food, either mingled with something belonging principally to the truth of human nature in the begetter, according to some, or without any such admixture, as others maintain. And thus the nutrimental humor in one becomes the radical humor in another.
The third opinion is that something new begins to belong principally to the truth of human nature even in this individual, because distinction in the human body does not require that any signate material part must needs remain throughout the whole lifetime; any signate part one may take is indifferent to this, whereas it remains always as regards what belongs to the species in it, albeit as regards what is material therein it may ebb and flow. And thus the nutrimental humor is not distinct from the radical on the part of its principle (so that it be called radical when begotten of the seed, and nutrimental when produced by the food), but rather on the part of the term, so that it be called radical when it reaches the term of generation by the act of the generative, or even nutritive power, but nutrimental, when it has not yet reached this term, but is still on the way to give nourishment.
These three opinions have been more fully exposed and examined in the Second Book (Sent. ii, D, 30); wherefore there is no need for repetition here, except in so far as the question at issue is concerned. It must accordingly be observed that this question requires different answers according to these opinions.
For the first opinion on account of its explanation of the process of multiplication is able to admit perfection of the truth of human nature, both as regards the number of individuals and as regards the due quantity of each individual, without taking into account that which is produced from food; for this is not added except for the purpose of resisting the destruction that might result from the action of natural heat, as lead is added to silver lest it be destroyed in melting. Wherefore since at the resurrection it behooves human nature to be restored to its perfection, nor does the natural heat tend to destroy the natural humor, there will be no need for anything resulting from food to rise again in man, but that alone will rise again which belonged to the truth of the human nature of the individual, and this reaches the aforesaid perfection in number and quantity by being detached and multiplied.
The second opinion, since it maintains that what is produced from food is needed for the perfection of quantity in the individual and for the multiplication that results from generation, must needs admit that something of this product from food shall rise again: not all, however, but only so much as is required for the perfect restoration of human nature in all its individuals. Hence this opinion asserts that all that was in the substance of the seed will rise again in this man who was begotten of this seed; because this belongs chiefly to the truth of human nature in him: while of that which afterwards he derives from nourishment, only so much will rise again in him as is needed for the perfection of his quantity; and not all, because this does not belong to the perfection of human nature, except in so far as nature requires it for the perfection of quantity. Since however this nutrimental humor is subject to ebb and flow the restoration will be effected in this order, that what first belonged to the substance of a man's body, will all be restored, and of that which was added secondly, thirdly, and so on, as much as is required to restore quantity. This is proved by two reasons. First, because that which was added was intended to restore what was wasted at first, and thus it does not belong principally to the truth of human nature to the same extent as that which came first. Secondly, because the addition of extraneous humor to the first radical humors results in the whole mixture not sharing the truth of the specific nature as perfectly as the first did: and the Philosopher instances as an example (De Gener. i) the mixing of water with wine, which always weakens the strength of the wine, so that in the end the wine becomes watery: so that although the second water be drawn into the species of wine, it does not share the species of wine as perfectly as the first water added to the wine. Even so that which is secondly changed from food into flesh does not so perfectly attain to the species of flesh as that which was changed first, and consequently does not belong in the same degree to the truth of human nature nor to the resurrection. Accordingly it is clear that this opinion maintains that the whole of what belongs to the truth of human nature principally will rise again, but not the whole of what belongs to the truth of human nature secondarily.
The third opinion differs somewhat from the second and in some respects agrees with it. It differs in that it maintains that whatever is under the form of flesh and bone all belongs to the truth of human nature, because this opinion does not distinguish as remaining in man during his whole lifetime any signate matter that belongs essentially and primarily to the truth of human nature, besides something ebbing and flowing, that belongs. to the truth of human nature merely on account of the perfection of quantity, and not on account of the primary being of the species, as the second opinion asserted. But it states that all the parts that are not beside the intention of the nature generated belong to the truth of human nature, as regards what they have of the species, since thus they remain; but not as regards what they have of matter, since thus they are indifferent to ebb and flow: so that we are to understand that the same thing happens in the parts of one man as in the whole population of a city, for each individual is cut off from the population by death, while others take their place: wherefore the parts of the people flow back and forth materially, but remain formally, since these others occupy the very same offices and positions from which the former were withdrawn, so that the commonwealth is said to remain the selfsame. In like manner, while certain parts are on the ebb and others are being restored to the same shape and position, all the parts flow back and forth as to their matter, but remain as to their species; and nevertheless the selfsame man remains.
On the other hand, The third opinion agrees with the second, because it holds that the parts which come secondly do not reach the perfection of the species so perfectly as those which come first: and consequently the third opinion asserts that the same thing rises again in man as the second opinion maintains, but not for quite the same reason. For it holds that the whole of what is produced from the seed will rise again, not because it belongs to the truth of human nature otherwise than that which comes after, but because it shares the truth of human nature more perfectly: which same order the second opinion applied to those things that are produced afterwards from food, in which point also these two opinions agree.