20 Yet many, with the art by which they seek to prove their heresy, are wont to delude the ears of the unlearned with the error, that as the body and soul of Adam both sinned, so the Lord must have taken the soul and body of Adam from the Virgin, and that it was not the whole Man that she conceived from the Holy Ghost. If they had understood the mystery of the Incarnation, these men would have understood at the same time the mystery that the Son of Man is also Son of God. As if in receiving so much from the Virgin, He received from her His soul also; whereas though flesh is always born of flesh, every soul is the direct work of God.
Source: On the Trinity (New Advent)