What is a merciful heart?

And what is a merciful heart? It is the heart's burning for the sake of the entire creation, for men, for birds, for animals, for demons and for every created thing; and by the recollection and sight of them the eyes of a merciful man pour forth abundant tears. From the strong and vehement mercy which grips his heart and from his great compassion, his heart is humbled and he cannot bear to hear or see any injury or slight sorrow in creation.For this reason he continually offers up tearful prayer, 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 in his heart without measure in the likeness of God.

St. Isaac The Syrian