Leo himself clearly indicates how he means to be the prudent physician using preventative remedies for his people. Agents from Alexandria had brought with them the monophysite doctrine that only the nature of the divinity was in Christ, and the real nature of human flesh which he took from Mary was not in him. These newcomers peddled their doctrine in the Velabrum, the commercial quarter of Rome near the Tiber, and it was to the Roman workers there, in the Basilica of St. Anastasia, that Leo once again clearly states the doctrine of the Incarnation, inveighing against the heresies of Eutyches, of Photinus, of the Manichaeans, and of Apollinaris.
JUST AS IT IS the duty of skilled and prudent physicians both to prevent by remedies the sufferings of human weakness, and to teach how to avoid things contrary to good health, dearly beloved, so it is the duty of the pastor to take care that the evil of heresy does not harm the flock of the Lord, and to demonstrate how to avoid the wickedness of those wolves and robbers.
Our Holy Fathers have always discovered and rightly condemned the evil of heresy no matter how cleverly it concealed itself. And so it was not able to conceal from our solicitude, which we owe to your affection, the fact that certain Egyptians, principally traders, had come to the City. They were supporting those statements detestably admitted by heretics at Alexandria. These asserted that, in Christ, there was only the nature of the divinity; that he did not have, in his innermost depths, the real nature of human flesh which he took from the Blessed Virgin Mary. What wickedness to say that Christ's manhood was false and that God is able to suffer. We cannot have any doubt about the spirit or the plan by which they dare to do this.