I'm having a discussion on someone elses blog about the phrase 'Mary, Mother of God'. I read this section of Aquinas's Shorter Summa tonight:
"The error of Nestorius, who refused to acknowledge that Blessed Mary is the Mother of God, is likewise excluded. Both Creeds assert that the Son of God was born or was made flesh of the Virgin Mary. The woman of whom any person is born is called his mother, for the reason that she supplies the matter for human conception. Hence the Blessed Virgin Mary, who provided the matter for the conception of the Son of God, should be called the true mother of the Son of God. As far as the essence of motherhood is concerned, the energy whereby the matter furnished by a woman is formed, does not enter into the question. She who supplied the matter to be formed by the Holy Spirit is no less a mother than a women who supplies matter that is to be formed by the energy latent in male seed. If anyone insists on maintaining that the Blessed Virgin ought not to be called the Mother of God because flesh alone and not divinity was derived from her, as Nestorious contended, he clearly is not aware of what he is saying. A woman is not called a mother for the reason that everything that is in her child is derived from her. Man is made up of body and soul; and a man is what he is in virtue of his soul rather than in virtue of his body. But no man's soul is derived from his mother. The soul is either created by God directly, as the true doctrine has it, or, if it were produced by transplanting, as some have fancied, it would be derived from the father rather than from the mother. For in the generation of other animals, according to the teaching of the philosophers, the male gives the soul, the female gives the body.
Consequently, just as any woman is a mother from the fact that her child's body is derived from her, so the Blessed Virgin Mary ought to be called the Mother of God if the body of God is derived from her. But we have to hold that it is the body of God, if it is taken up into the unity of the person of God's Son, who is true God. Therefore all who admit that human nature was assumed by the Son of God into the unity of His person, must admit that the Blessed Virgin Mary is the Mother of God. But Nestorious, who denied that the person of God and of the man Jesus Christ was one, was forced by logical necessity to deny that the Virgin Mary was the Mother of God."
Clearly, St. Thomas was not a biologist, and he was working with the scientific knowledge of the time. But I find the argument interesting.