Objection 1. It would seem that the soul was not produced by creation. For that which has in itself something material is produced from matter. But the soul is in part material, since it is not a pure act. Therefore the soul was made of matter; and hence it was not created.
Objection 2. Further, every actuality of matter is educed from the potentiality of that matter; for since matter is in potentiality to act, any act pre-exists in matter potentially. But the soul is the act of corporeal matter, as is clear from its definition. Therefore the soul is educed from the potentiality of matter.
Objection 3. Further, the soul is a form. Therefore, if the soul is created, all other forms also are created. Thus no forms would come into existence by generation; which is not true.
On the contrary, It is written (Genesis 1:27): "God created man to His own image." But man is like to God in his soul. Therefore the soul was created.
I answer that, The rational soul can be made only by creation; which, however, is not true of other forms. The reason is because, since to be made is the way to existence, a thing must be made in such a way as is suitable to its mode of existence. Now that properly exists which itself has existence; as it were, subsisting in its own existence. Wherefore only substances are properly and truly called beings; whereas an accident has not existence, but something is (modified) by it, and so far is it called a being; for instance, whiteness is called a being, because by it something is white. Hence it is said Metaph. vii, Did. vi, 1 that an accident should be described as "of something rather than as something." The same is to be said of all non-subsistent forms. Therefore, properly speaking, it does not belong to any non-existing form to be made; but such are said to be made through the composite substances being made. On the other hand, the rational soul is a subsistent form, as above explained (75, 2). Wherefore it is competent to be and to be made. And since it cannot be made of pre-existing matter--whether corporeal, which would render it a corporeal being--or spiritual, which would involve the transmutation of one spiritual substance into another, we must conclude that it cannot exist except by creation.
Reply to Objection 1. The soul's simple essence is as the material element, while its participated existence is its formal element; which participated existence necessarily co-exists with the soul's essence, because existence naturally follows the form. The same reason holds if the soul is supposed to be composed of some spiritual matter, as some maintain; because the said matter is not in potentiality to another form, as neither is the matter of a celestial body; otherwise the soul would be corruptible. Wherefore the soul cannot in any way be made of pre-existent matter.
Reply to Objection 2. The production of act from the potentiality of matter is nothing else but something becoming actually that previously was in potentiality. But since the rational soul does not depend in its existence on corporeal matter, and is subsistent, and exceeds the capacity of corporeal matter, as we have seen (75, 2), it is not educed from the potentiality of matter.
Reply to Objection 3. As we have said, there is no comparison between the rational soul and other forms.
Source: Summa Theologica (New Advent)