Objection 1. It would seem that none of the food is changed into true human nature. For it is written (Matthew 15:17): "Whatsoever entereth into the mouth, goeth into the belly, and is cast out into the privy." But what is cast out is not changed into the reality of human nature. Therefore none of the food is changed into true human nature.
Objection 2. Further, the Philosopher (De Gener. i, 5) distinguishes flesh belonging to the "species" from flesh belonging to "matter"; and says that the latter "comes and goes." Now what is formed from food comes and goes. Therefore what is produced from food is flesh belonging to matter, not to the species. But what belongs to true human nature belongs to the species. Therefore the food is not changed into true human nature.
Objection 3. Further, the "radical humor" seems to belong to the reality of human nature; and if it be lost, it cannot be recovered, according to physicians. But it could be recovered if the food were changed into the humor. Therefore food is not changed into true human nature.
Objection 4. Further, if the food were changed into true human nature, whatever is lost in man could be restored. But man's death is due only to the loss of something. Therefore man would be able by taking food to insure himself against death in perpetuity.
Objection 5. Further, if the food is changed into true human nature, there is nothing in man which may not recede or be repaired: for what is generated in a man from his food can both recede and be repaired. If therefore a man lived long enough, it would follow that in the end nothing would be left in him of what belonged to him at the beginning. Consequently he would not be numerically the same man throughout his life; since for the thing to be numerically the same, identity of matter is necessary. But this is incongruous. Therefore the food is not changed into true human nature.
On the contrary, Augustine says (De Vera Relig. xi): "The bodily food when corrupted, that is, having lost its form, is changed into the texture of the members." But the texture of the members belongs to true human nature. Therefore the food is changed into the reality of human nature.
I answer that, According to the Philosopher (Metaph. ii), "The relation of a thing to truth is the same as its relation to being." Therefore that belongs to the true nature of any thing which enters into the constitution of that nature. But nature can be considered in two ways: firstly, in general according to the species; secondly, as in the individual. And whereas the form and the common matter belong to a thing's true nature considered in general; individual signate matter, and the form individualized by that matter belong to the true nature considered in this particular individual. Thus a soul and body belong to the true human nature in general, but to the true human nature of Peter and Martin belong this soul and this body.
Now there are certain things whose form cannot exist but in one individual matter: thus the form of the sun cannot exist save in the matter in which it actually is. And in this sense some have said that the human form cannot exist but in a certain individual matter, which, they said, was given that form at the very beginning in the first man. So that whatever may have been added to that which was derived by posterity from the first parent, does not belong to the truth of human nature, as not receiving in truth the form of human nature.
But, said they, that matter which, in the first man, was the subject of the human form, was multiplied in itself: and in this way the multitude of human bodies is derived from the body of the first man. According to these, the food is not changed into true human nature; we take food, they stated, in order to help nature to resist the action of natural heat, and prevent the consumption of the "radical humor"; just as lead or tin is mixed with silver to prevent its being consumed by fire.
But this is unreasonable in many ways.
Firstly, because it comes to the same that a form can be produced in another matter, or that it can cease to be in its proper matter; wherefore all things that can be generated are corruptible, and conversely. Now it is manifest that the human form can cease to exist in this (particular) matter which is its subject: else the human body would not be corruptible. Consequently it can begin to exist in another matter, so that something else be changed into true human nature.
Secondly, because in all beings whose entire matter is contained in one individual there is only one individual in the species: as is clearly the case with the sun, moon and such like. Thus there would only be one individual of the human species.
Thirdly, because multiplication of matter cannot be understood otherwise than either in respect of quantity only, as in things which are rarefied, so that their matter increases in dimensions; or in respect of the substance itself of the matter. But as long as the substance alone of matter remains, it cannot be said to be multiplied; for multitude cannot consist in the addition of a thing to itself, since of necessity it can only result from division. Therefore some other substance must be added to matter, either by creation, or by something else being changed into it. Consequently no matter can be multiplied save either by rarefaction as when air is made from water; or by the change of some other things, as fire is multiplied by the addition of wood; or lastly by creation. Now it is manifest that the multiplication of matter in the human body does not occur by rarefaction: for thus the body of a man of perfect age would be more imperfect than the body of a child. Nor does it occur by creation of flesh matter: for, according to Gregory (Moral. xxxii): "All things were created together as to the substance of matter, but not as to the specific form." Consequently the multiplication of the human body can only be the result of the food being changed into the true human nature.
Fourthly, because, since man does not differ from animals and plants in regard to the vegetative soul, it would follow that the bodies of animals and plants do not increase through a change of nourishment into the body so nourished, but through some kind of multiplication. Which multiplication cannot be natural: since the matter cannot naturally extend beyond a certain fixed quantity; nor again does anything increase naturally, save either by rarefaction or the change of something else into it. Consequently the whole process of generation and nourishment, which are called "natural forces," would be miraculous. Which is altogether inadmissible.
Wherefore others have said that the human form can indeed begin to exist in some other matter, if we consider the human nature in general: but not if we consider it as in this individual. For in the individual the form remains confined to a certain determinate matter, on which it is first imprinted at the generation of that individual, so that it never leaves that matter until the ultimate dissolution of the individual. And this matter, say they, principally belongs to the true human nature. But since this matter does not suffice for the requisite quantity, some other matter must be added, through the change of food into the substance of the individual partaking thereof, in such a quantity as suffices for the increase required. And this matter, they state, belongs secondarily to the true human nature: because it is not required for the primary existence of the individual, but for the quantity due to him. And if anything further is produced from the food, this does not belong to true human nature, properly speaking. However, this also is inadmissible.
First, because this opinion judges of living bodies as of inanimate bodies; in which, although there be a power of generating their like in species, there is not the power of generating their like in the individual; which power in living bodies is the nutritive power. Nothing, therefore, would be added to living bodies by their nutritive power, if their food were not changed into their true nature.
Secondly, because the active seminal power is a certain impression derived from the soul of the begetter, as stated above (Question 118, Article 1). Hence it cannot have a greater power in acting, than the soul from which it is derived. If, therefore, by the seminal power a certain matter truly assumes the form of human nature, much more can the soul, by the nutritive power, imprint the true form of human nature on the food which is assimilated.
Thirdly, because food is needed not only for growth, else at the term of growth, food would be needful no longer; but also to renew that which is lost by the action of natural heat. But there would be no renewal, unless what is formed from the food, took the place of what is lost. Wherefore just as that which was there previously belonged to true human nature, so also does that which is formed from the food.
Therefore, according to others, it must be said that the food is really changed into the true human nature by reason of its assuming the specific form of flesh, bones and such like parts. This is what the Philosopher says (De Anima ii, 4): "Food nourishes inasmuch as it is potentially flesh."
Reply to Objection 1. Our Lord does not say that the "whole" of what enters into the mouth, but "all"--because something from every kind of food is cast out into the privy. It may also be said that whatever is generated from food, can be dissolved by natural heat, and be cast aside through the pores, as Jerome expounds the passage.
Reply to Objection 2. By flesh belonging to the species, some have understood that which first receives the human species, which is derived from the begetter: this, they say, lasts as long as the individual does. By flesh belonging to the matter these understand what is generated from food: and this, they say, does not always remain, but as it comes so it goes. But this is contrary to the mind of Aristotle. For he says there, that "just as in things which have their species in matter"--for instance, wood or stone--"so in flesh, there is something belonging to the species, and something belonging to matter." Now it is clear that this distinction has no place in inanimate things, which are not generated seminally, or nourished. Again, since what is generated from food is united to, by mixing with, the body so nourished, just as water is mixed with wine, as the Philosopher says there by way of example: that which is added, and that to which it is added, cannot be different natures, since they are already made one by being mixed together. Therefore there is no reason for saying that one is destroyed by natural heat, while the other remains.
It must therefore be said that this distinction of the Philosopher is not of different kinds of flesh, but of the same flesh considered from different points of view. For if we consider the flesh according to the species, that is, according to that which is formed therein, thus it remains always: because the nature of flesh always remains together with its natural disposition. But if we consider flesh according to matter, then it does not remain, but is gradually destroyed and renewed: thus in the fire of a furnace, the form of fire remains, but the matter is gradually consumed, and other matter is substituted in its place.
Reply to Objection 3. The "radical humor" is said to comprise whatever the virtue of the species is founded on. If this be taken away it cannot be renewed; as when a man's hand or foot is amputated. But the "nutritive humor" is that which has not yet received perfectly the specific nature, but is on the way thereto; such is the blood, and the like. Wherefore if such be taken away, the virtue of the species remains in its root, which is not destroyed.
Reply to Objection 4. Every virtue of a passible body is weakened by continuous action, because such agents are also patient. Therefore the transforming virtue is strong at first so as to be able to transform not only enough for the renewal of what is lost, but also for growth. Later on it can only transform enough for the renewal of what is lost, and then growth ceases. At last it cannot even do this; and then begins decline. In fine, when this virtue fails altogether, the animal dies. Thus the virtue of wine that transforms the water added to it, is weakened by further additions of water, so as to become at length watery, as the Philosopher says by way of example (De Gener. i, 5).
Reply to Objection 5. As the Philosopher says (De Gener. i, 5), when a certain matter is directly transformed into fire, then fire is said to be generated anew: but when matter is transformed into a fire already existing, then fire is said to be fed. Wherefore if the entire matter together loses the form of fire, and another matter transformed into fire, there will be another distinct fire. But if, while one piece of wood is burning, other wood is laid on, and so on until the first piece is entirely consumed, the same identical fire will remain all the time: because that which is added passes into what pre-existed. It is the same with living bodies, in which by means of nourishment that is renewed which was consumed by natural heat.
Source: Summa Theologica (New Advent)