Thomas Aquinas, Compendium of Theology

Certain considerations seem opposed to our doctrine. In the first place, specific difference is to genus what form is to matter. Animal is the genus of man, and rational is the difference that makes man what he is. Accordingly, since animal is a body animated by a sensitive soul, it seems that a body animated by a sensitive soul is still in potency with respect to the rational soul. Thus the rational soul would be distinct from the sensitive soul.

Moreover, the intellect does not possess a bodily organ. But the sensitive and nutritive powers do possess bodily organs. Hence it seems impossible for the same soul to be both intellectual and sensitive, because the same thing cannot both be separated and not separated from another thing.

Furthermore, the rational soul is incorruptible, as was shown above. On the other hand, the vegetative and the sensitive souls are corruptible, as they are acts of corruptible organs. Therefore the rational soul is not the same as the vegetative and the sensitive souls, for the same thing cannot be both corruptible and incorruptible.

Besides, in the generation of man the life conferred by the vegetative soul appears before the fetus is observed to be an animal from its sense activity and motion; and this same being is discerned to be an animal through its sense activity and movement before it has an intellect. Therefore, if the soul by which the fetus first lives the life of a plant, then the life of an animal, and thirdly the life of a man, is the same, it would follow that the vegetative, sensitive, and rational principles come from an outside source, or else that the intellectual soul arises from the energy in the semen. Both of these alternatives are inadmissible. On the one hand, since the operations of the vegetative and sensitive soul are not exercised apart from the body, their principles cannot be without a body. On the other hand, the operation of the intellectual soul is exercised without a body; and so, apparently, no bodily energy can be its cause. Therefore the same soul cannot be vegetative, sensitive, and rational.