Justin Martyr on the Causative Force behind the Virgin Birth

Greetings,

What caused the Virgin to conceive and give birth to Jesus of Nazareth? How was a woman "not knowing any man" able to have a child? Here is Justin Martyr's reasoning on the matter:

"But lest some, not understanding the prophecy now
cited [Isa 7:14], should charge us with the very
things we have been laying to the charge of the poets
who say that Jupiter went in to women through lust,
let us try to explain the words. This, then, 'Behold,
a virgin shall conceive,' signifies that a virgin
should conceive without intercourse. For if she had
had intercourse with any one whatever, she was no
longer a virgin; but the power of God having come upon
the virgin, overshadowed her, and caused her while yet
a virgin to conceive. And the angel of God who was
sent to the same virgin at that time brought her good
news, saying, 'Behold, thou shalt conceive of the Holy
Ghost, and shalt bear a Son, and He shall be called
the Son of the Highest, and thou shalt call His name
Jesus; for He shall save His people from their
sins,'--as they who have recorded all that concerns
our Saviour Jesus Christ have taught, whom we
believed, since by Isaiah also, whom we have now
adduced, the Spirit of prophecy declared that He
should be born as we intimated before. It is wrong,
therefore, to understand the Spirit and the power of
God as anything else than the Word, who is also the
first-born of God, as the foresaid prophet Moses
declared; and it was this which, when it came upon the
virgin and overshadowed her, caused her to conceive,
not by intercourse, but by power" (1 Apology 33).

"But who, through the power of the Word, according to
the will of God the Father and Lord of all, He was
born of a virgin as a man, and was named Jesus, and
was crucified, and died, and rose again, and ascended
into heaven, an intelligent man will be able to
comprehend from what has been already so largely said"
(1 Apology 46).