And what is a merciful heart? It is the heart burning for the sake of all creation, for men, for birds, for animals, for demons, and every created thing; and by the recollection of them the eyes of a merciful man pour forth abundant tears. By the strong and vehement mercy which grips his heart and by his great compassion, his heart is humbled and he cannot bear to hear or to see any injury or slight sorrow in creation. For this reason he offers up tearful prayer continually even for irrational beasts, for the enemies of the truth, and for those who harm him, that they be protected and receive mercy. And in like manner he even prays for the family of reptiles because of the great compassion that burns without measure in his heart in the likeness of God.
– St. Isaac the Syrian (d. 700)