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 Obj. 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 Obj. 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 Obj. 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 Obj. 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 Obj. 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. _______________________
SECOND ARTICLE [I, Q. 119, Art. 2]
Whether the Semen Is Produced from Surplus Food?
Objection 1: It would seem that the semen is not produced from the surplus food, but from the substance of the begetter. For Damascene says (De Fide Orth. i, 8) that "generation is a work of nature, producing, from the substance of the begetter, that which is begotten." But that which is generated is produced from the semen. Therefore the semen is produced from the substance of the begetter.