19.
This is the plain drift and design of the prophet, literally, obviously, and primarily understood; and thus he is understood by one of the most judicious of interpreters, the great Grotius.

Indeed, to understand the prophet as having the conception of Mary, and the birth of her son Jesus from a virgin mother literally, and primarily in view, is a very great absurdity, and contrary to the very intent and design of the sign given by the prophet.
For the sign being given by Isaiah to convince Ahaz that he brought a message from God to him, to assure him that the two kings should not succeed in their attempt against him, how could a virgin's conception, and bearing a son seven hundred years afterwards, be a sign to Ahaz, that the prophet came to him, with the said message from God?
And how useless was it to Ahaz, as well as absurd in itself for the prophet, to say, "Before the child, born seven hundred years hence, shall distinguish between good and evil, the land which thou abhorrest shall be forsaken of both her kings," which would be a banter, instead of a sign.
But a prophecy of the certain birth of a male child, by a particular female within a short time, seems a proper sign, as being not only what could not with certainty, be foretold, except by a person inspired, but considered as soon coming to pass, it, consequently, evidences itself to be a divine sign, and answers all the purposes of a sign.

And such a sign is agreeable to God's conduct on like occasions; witness his conduct to Gideon and Hezekiah.Jud.

vi.; 2 Kings xx.
This prophecy, therefore, not being fulfilled in Jesus, according to the literal and obvious sense of the words as they stand in Isaiah, it is supposed that this, like the other prophecies cited in the New Testament, is fulfilled in a secondary, or typical, or mystical sense; that is, the said prophecy, which was literally fulfilled by the birth of the son foretold by the prophet, was again fulfilled by the birth of Jesus, as being an event of the same kind, and intended to be secretly and mystically signified either by the prophet or by God, who directed the prophet's speech.

If the reader desires further satisfaction that the literal and obvious sense of this prophecy relates to a son to be born in Isaiah's time, and not to Jesus, he is referred to the commentator Grotius, and to Huetius' Demonstrat.
Evang.