I. The Faith is the mean between the two extremes of Eutyches and Nestorius.
Leo to Maximus of Antioch.
How much, beloved, you have at heart the most sacred unity of our common Faith and the tranquil harmony of the Church's peace, the substance of your letter shows, which was brought me by our sons, Marian the presbyter and Olympius the deacon, and which was the more welcome to us because thereby we can join as it were in conversation, and thus the grace of God becomes more and more known and greater joy is felt through the whole world over the revelation of Catholic Truth. And yet we are sore grieved at some who still (so your messengers indicate) love their darkness; and though the brightness of day has arisen everywhere, even still delight in the obscurity of their blindness, and abandoning the Faith, remain Christians in only the empty name, without knowledge to discern one error from another, and to distinguish the blasphemy of Nestorius from the impiety of Eutyches. For no delusion of theirs can appear excusable, because they contradict themselves in their perverseness. For, though Eutyches' disciples abhor Nestorius, and the followers of Nestorius anathematize Eutyches, yet in the judgment of Catholics both sides are condemned and both heresies alike are cut away from the body of the Church: because neither falsehood can be in unison with us. Nor does it matter in which direction of blasphemy they disagree with the truth of the Lord's Incarnation, since their erroneous opinions hold neither with the authority of the Gospel nor with the significance of the mystery.
Source: Letters (New Advent)