The Most Ancient Lives of Saint Patrick/The Life and Acts of St. Patrick/Chapter 154
A Stone is changed into Milk, and Milk is changed into Stones.
And one who had long time been a servant unto many evil-doers, hearing of the virtues and the miracles of Saint Patrick, came unto him, for the purpose of contending with him in working signs. And many false signs did he multiply, the which the saint, having prayed and made the sign of the cross, dispersed. Then the magician seeing all his inventions to be frustrated, required of Patrick that he should work signs to evince the power of his God; and the saint delayed not to do what might prove the virtue of Christ, and instruct in the faith many Christians: for he changed an hard stone into a soft mass of curdled milk, and of this milk, in the name of Christ, he changed two soft pieces into hard stones. But lest these should be accounted false and like unto the signs of the magicians, the stones continued in the same hardness whereunto they were transformed. But this which was corporally done before the eyes of men, doth the divine virtue spiritually do in the conversion of believers; inasmuch as the worshippers of stones, men of hardened hearts, become soft unto the faith and love of Christ, and as if again born infants, they desire the milk of the apostolic doctrine, that thereby they may grow up unto salvation. So did it happen unto the magician, who beholding this miracle believed in the Lord and was baptized.