Objection 1. It would seem that the human soul was made before the body. For the work of creation preceded the work of distinction and adornment, as shown above (66, 1; 70, 1). But the soul was made by creation; whereas the body was made at the end of the work of adornment. Therefore the soul of man was made before the body.
Objection 2. Further, the rational soul has more in common with the angels than with the brute animals. But angels were created before bodies, or at least, at the beginning with corporeal matter; whereas the body of man was formed on the sixth day, when also the animals were made. Therefore the soul of man was created before the body.
Objection 3. Further, the end is proportionate to the beginning. But in the end the soul outlasts the body. Therefore in the beginning it was created before the body.
On the contrary, The proper act is produced in its proper potentiality. Therefore since the soul is the proper act of the body, the soul was produced in the body.
I answer that, Origen (Peri Archon i, 7,8) held that not only the soul of the first man, but also the souls of all men were created at the same time as the angels, before their bodies: because he thought that all spiritual substances, whether souls or angels, are equal in their natural condition, and differ only by merit; so that some of them--namely, the souls of men or of heavenly bodies--are united to bodies while others remain in their different orders entirely free from matter. Of this opinion we have already spoken (47, 2); and so we need say nothing about it here.
Augustine, however (Gen. ad lit. vii, 24), says that the soul of the first man was created at the same time as the angels, before the body, for another reason; because he supposes that the body of man, during the work of the six days, was produced, not actually, but only as to some "causal virtues"; which cannot be said of the soul, because neither was it made of any pre-existing corporeal or spiritual matter, nor could it be produced from any created virtue. Therefore it seems that the soul itself, during the work of the six days, when all things were made, was created, together with the angels; and that afterwards, by its own will, was joined to the service of the body. But he does not say this by way of assertion; as his words prove. For he says (Gen. ad lit. vii, 29): "We may believe, if neither Scripture nor reason forbid, that man was made on the sixth day, in the sense that his body was created as to its causal virtue in the elements of the world, but that the soul was already created."
Now this could be upheld by those who hold that the soul has of itself a complete species and nature, and that it is not united to the body as its form, but as its administrator. But if the soul is united to the body as its form, and is naturally a part of human nature, the above supposition is quite impossible. For it is clear that God made the first things in their perfect natural state, as their species required. Now the soul, as a part of human nature, has its natural perfection only as united to the body. Therefore it would have been unfitting for the soul to be created without the body.
Therefore, if we admit the opinion of Augustine about the work of the six days (74, 2), we may say that the human soul preceded in the work of the six days by a certain generic similitude, so far as it has intellectual nature in common with the angels; but was itself created at the same time as the body. According to the other saints, both the body and soul of the first man were produced in the work of the six days.
Reply to Objection 1. If the soul by its nature were a complete species, so that it might be created as to itself, this reason would prove that the soul was created by itself in the beginning. But as the soul is naturally the form of the body, it was necessarily created, not separately, but in the body.
Reply to Objection 2. The same observation applies to the second objection. For if the soul had a species of itself it would have something still more in common with the angels. But, as the form of the body, it belongs to the animal genus, as a formal principle.
Reply to Objection 3. That the soul remains after the body, is due to a defect of the body, namely, death. Which defect was not due when the soul was first created.
Source: Summa Theologica (New Advent)