The Word could not be defiled by the womb
Augustine is answering the objection that Christ's birth from a woman's womb would be unclean, arguing that the incorporeal Word could not be polluted by a human body.
Nor should the thought of a woman's inner parts weaken this faith of ours, as if the generation of our Lord must be rejected because the unclean ones think it unclean. For "the foolishness of God is wiser than men" (1 Corinthians 1:25), and the apostle speaks most truly when he says, "all things are clean to the clean" (Titus 1:15). Those who think this way ought to look at the rays of this sun, which they certainly do not praise as a creature of God but worship as a god. Those rays are poured out everywhere through the stench of sewers and every kind of horror, and they work in these places according to their nature, yet they are not soiled by any contamination from them—even though visible light is by nature more closely joined to visible filth. How much less, then, could the Word of God, which is not bodily or visible, be polluted by a woman's body, where he took up human flesh together with soul and spirit? Through these intervening parts the majesty of the Word dwells more hidden from the frailty of the human body. From this it is clear that the Word of God could in no way be stained by a human body, since not even the human soul itself is stained by it. For the soul is not stained by the body when it rules the body and gives it life, but when it desires the body's good mortal things.
Latin
Nec nobis fidem istam minuat cogitatio muliebrium uiscerum, ut propterea recusanda uideatur talis domini nostri generatio, quod eam sordidi sordidam putant, quia et 'stultum dei sapientius est hominibus> et (omnia munda mundis* uerissime apostolus dicit. debent igitur intueri, qui hoc putant, solis huius radios, quem certe non tamquam creaturam dei laudant, sed tamquam deum adorant, per cloacarum fetores et quaeque horribilia usquequaque diffundi et in his operari secundum naturam suam nec tamen inde aliqua contaminatione sordescere, cum uisibilis lux uisibilibus sordibus sit natura coniunctior: quanto minus igitur poterat pollui uerbum dei non corporeum neque uisibile de femineo corpore, ubi humanam carnem suscepit cum anima et spiritu, quibus interuenientibus habitat maiestas uerbi ab humani corporis fragilitate secretius! unde manifestum est nullo modo potuisse uerbum dei maculari humano corpore, quo nec ipsa anima humana maculata est. non enim cum regit corpus atque uiuificat, sed cum eius bona mortalia concupiscit, de corpore anima maculatur.