Thomas Aquinas

“ But it is clear that the common is to the proper and determinate, as imperfect is to perfect: therefore we see that in the generation of an animal, the animal is generated first, then the man or the horse. So therefore food first of all receives a certain common virtue in regard to all the parts of the body, which virtue is subsequently determinate to this or that part.
Now it is not possible that the semen be a kind of solution from what is already transformed into the substance of the members. ”

“ 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. ”

“ 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. ”

“ Further, the son is like his father, in respect of that which he receives from him. But if the semen from which something is generated, is produced from the surplus food, a man would receive nothing from his grandfather and his ancestors in whom the food never existed. ”

“ 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 ”

“ Now it belongs to the natural order that a thing should be reduced from potentiality to act gradually: hence in things generated we observe that at first each is imperfect and is afterwards perfected. ”