Thomas Aquinas, Compendium of Theology

This ultimate and complete form, the rational soul, is brought into existence, not by the power that is in the semen, but by a higher cause. For the power that is in the semen is a bodily power. But the rational soul exceeds the whole nature and power of the body, since no body can rise to the heights of the soul's intellectual activity. Nothing can act in a way that surmounts its species, because the agent is nobler than the patient, and the maker excels his product. Hence the power possessed by a body cannot produce the rational soul, nor, consequently, can the energy inherent in the semen do so.

Moreover, a thing that has new existence must also have a new becoming; for that which is, must first become, since a thing becomes in order that it may be. Thus things which have being in their own right must have becoming in their own right; such are subsistent beings. But things that do not possess being in their own right do not properly have a becoming; such are accidents and material forms. The rational soul has being in its own right, because it has its own operation, as is clear from our previous discussion. Therefore becoming is properly predicated of the rational soul. Since the soul is not composed of matter and form, as was shown above, it cannot be brought into being except by creation. But God alone can create, as we said above. Consequently the rational soul is produced by God alone.

We can readily understand why this should be so. In all arts that are hierarchically related to one another, we observe that the highest art induces the ultimate form, whereas the lower arts dispose matter for the reception of the ultimate form. The rational soul, evidently, is the ultimate and most perfect form that the matter of beings subject to generation and corruption can achieve. Therefore natural agents, which operate on lower levels, appropriately cause preliminary dispositions and forms, whereas the supreme agent, God, causes the ultimate form, which is the rational soul.