I think, then, that it has been pretty well established not only that our Saviour was to be born of a virgin, but also that there were prophets among the Jews who uttered not merely general predictions about the future—as, e.g., regarding Christ and the kingdoms of the world, and the events that were to happen to Israel, and those nations which were to believe in the Saviour, and many other things concerning Him—but also prophecies respecting particular events; as, for instance, how the asses of Kish, which were lost, were to be discovered, and regarding the sickness which had fallen upon the son of the king of Israel, and any other recorded circumstance of a similar kind.
But as a further answer to the Greeks, who do not believe in the birth of Jesus from a virgin, we have to say that the Creator has shown, by the generation of several kinds of animals, that what He has done in the instance of one animal, He could do, if it pleased Him, in that of others, and also of man himself. For it is ascertained that there is a certain female animal which has no intercourse with the male (as writers on animals say is the case with vultures), and that this animal, without sexual intercourse, preserves the succession of race.
What incredibility, therefore, is there in supposing that, if God wished to send a divine teacher to the human race, He caused Him to be born in some manner different from the common! Nay, according to the Greeks themselves, all men were not born of a man and woman. For if the world has been created, as many even of the Greeks are pleased to admit, then the first men must have been produced not from sexual intercourse, but from the earth, in which spermatic elements existed; which, however, I consider more incredible than that Jesus was born like other men, so far as regards the half of his birth.
And there is no absurdity in employing Grecian histories to answer Greeks, with the view of showing that we are not the only persons who have recourse to miraculous narratives of this kind. For some have thought fit, not in regard to ancient and heroic narratives, but in regard to events of very recent occurrence, to relate as a possible thing that Plato was the son of Amphictione, Ariston being prevented from having marital intercourse with his wife until she had given birth to him with whom she was pregnant by Apollo.
And yet these are veritable fables, which have led to the invention of such stories concerning a man whom they regarded as possessing greater wisdom and power than the multitude, and as having received the beginning of his corporeal substance from better and diviner elements than others, because they thought that this was appropriate to persons who were too great to be human beings. And since Celsus has introduced the Jew disputing with Jesus, and tearing in pieces, as he imagines, the fiction of His birth from a virgin, comparing the Greek fables about Danaë, and Melanippe, and Auge, and Antiope, our answer is, that such language becomes a buffoon, and not one who is writing in a serious tone.
Source: Against Celsus (New Advent)