13 But there is another consideration which we must not leave out in the solution of this question, namely, that the substance of God, which is wholly incorporeal, cannot be introduced into bodies or be received by them in the first instance, unless there be some spiritual substance as a medium, which is capable of receiving the divine Spirit. For instance, if we say that light is able to irradiate all the members of the body, yet by none of them can it be received except by the eye.
For it is the eye alone which is receptive of light. So the Son of God is born of a virgin, not associated with the flesh alone in the first instance, but begotten with a soul as a medium between the flesh and God. With the soul, then, serving as a medium, and receiving the Word of God in the secret citadel of the rational spirit, God was born of the Virgin without any such disparagement as you imagine. And therefore nothing is to be esteemed base or unseemly wherein was the sanctification of the Spirit, and where the soul which was capable of God became also a partaker of flesh. Account nothing impossible where the power of the Most High was present. Have no thought of human weakness where there was the plenitude of Divinity.
Source: Commentary on the Apostles' Creed (New Advent)