Seed and soil
The people of Antiquity subscribed to what is called the “seed and soil” theory of human reproduction. According to this theory the father conveyed life and form to the child in and through his seed. As with a plant seed then, most if not all morphological features were passed on by the father’s σπέρμα. The mother, on the other hand, provided the matter and nutrients out of which the seed could grow. As Andrew Lincoln explains:
Their understanding of conception, shaped by a patriarchal culture, would have been some variation of the dominant Aristotelian theory. On this view, the male semen provides the formative principle for life. The female menstrual blood supplies the matter for the fetus, and the womb the medium for the semen’s nurture. The man’s seed transmits his logos (rational cause) and pneuma (vital heat/animating spirit), for which the woman’s body is the receptacle. In this way the male functions as the active, efficient cause of reproduction, and the female functions as the provider of the matter to which the male seed gives definition. In short, the bodily substance necessary for a human fetus comes from the mother, while the life force originates with the father…
Jewish scriptural writings of the time assume the dominant biological understanding that the male seed is implanted in the womb and gives life to the blood, the substance contributed by the mother. The contemporaneous Jewish philosopher Philo states, “The material of the female is supplied to the son from what remains over the eruption of blood, while the immediate make and cause of the son is the male.” Later rabbinic traditions concerning embryology and procreation were deeply embedded within this Greco-Roman tradition, as were patristic views. This ancient understanding continued to be held by Thomas Aquinas: “The female supplies the matter, while the male is the active principle of generation.”
“How Babies Were Made in Jesus’ Time”, Andrew Lincoln, (BAR Nov/Dec 2014)
This ancient understanding of procreation is found in the Biblical context most readily in the second creation account. According to the text of Genesis 2, God as father actively molds (πλάσσω) the human being into form as a potter would an idol. He then breathes (ἐμφυσάω) into the idol the breath of life (πνοή ζωῆς) such that it becomes a living being (ψυχὴν ζῶσαν). The earth as mother, however, is the passive agent of creation. From her (ἀπὸ τῆς γῆς) comes the loose dirt (χοῦς) with which the craftsman creates the idol. The father thus gives the earthling his shape and his animation; the mother gives him his substance.
Begotten from holy spirit
Strange as it may seem in light of modern science, this pragmatic and patriarchal seed and soil theory of reproduction prevailed over the Classical world in which the first Christians lived. The initial readers of the Gospels were therefore constrained to interpret Christ’s conception through the lens that such a theory provided.
The implications of this insight are twofold I believe.
It first means that Christ’s material substance came wholly from his mother. As all mothers do, Mary provided the inert blood and dust that came to make up her son’s body.
It secondly means that God, like all fathers, formed Mary’s substance into its shape, giving Jesus both his physical form and his personhood—his reason, will, energy, and life. Yet unlike human fathers, God did all this not through his seed but through his spirit. In like manner, God was about to “beget” believers not “out of perishable seed” (ἐκ σπορᾶς φθαρτῆς) but out of the imperishable seed that is the Father’s spirit (1 Peter 1:23) This spirit as seed metaphor is common in Paul (cf. Galatians 5:22, 6:8, 1 Corinthians 9:11).
What we encounter in Christ’s virginal conception then is God directly “conceiving” a man from the dirt, in this case Mary’s womb. God once again “begets” adam from his own “seed,” his πνεῦμα, rather than from the seed of man. This seed-like divine πνεῦμα comes upon and overshadows the soil provided by Mary.
Jesus son of David
From here I think we are in a better position to comprehend a major problem with Christ’s ancestry: How can Christ be the son of David and yet not be begotten from the seed of David’s descendant, Joseph?
Setting aside the more conventional answers for a moment, it seems to me that the soil and seed theory offers us a way forward: God as father formed Jesus into the image of David through the Holy Spirit rather than through Joseph’s Davidic seed. Just as Joseph could have passed on David’s character and form to Jesus by means of his own seed (ἐκ σπέρματος Δαβὶδ), God passed on the Davidic image to Jesus by means of his holy spirit (ἐκ πνεύματος ἁγίου). In this God fulfilled the active paternal role of procreation and secured Jesus’ identity as Davidic Messiah.
Yet at this point other questions arise: Why does God transmit the Davidic image to his son? Why is Jesus not only the son of God but also the son of David through his spiritual conception (cf. Luke 1:31-32, Matthew 1:1, 2:15)? Or, put another way, if a father passes on his own image to his son (cf. Genesis 5:3), why is God’s son David’s son?
It seems that for the early Christians the Davidic form ultimately had its origins in God, not in man. For distilled down to its essence, the Davidic image was seen as God’s image impressed upon a human being. David was the man “according to the Lord’s heart” (κατὰ τὴν καρδίαν αὐτοῦ) after all. He was remembered and defined by his likeness to God. For this reason the titles son of David and son of God blurred together during the second temple period (cf. 2 Samuel 7:14, Psalm 89:26, 4Q246, Didache 9:2). And so God’s son would be like David because David bore God’s image and likeness.