And what is a merciful heart? … The heart’s burning for all creation, for human beings, for birds and animals, and for demons, and everything there is. At the recollection of them and at the sight of them his eyes gush forth with tears owing to the force of the compassion which constrains his heart, so that, as a result of its abundant sense of mercy, the heart shrinks and cannot bear to hear or examine any harm or small suffering of anything in creation.
For this reason he offers up prayer with tears at all times, even for irrational animals, and for the enemies of truth, and for those who harm him, for their preservation and being forgiven … as a result of the immense compassion infused in his heart without measure – like God’s.
– Isaac of Nineveh