“What is a charitable heart? It is a heart which is burning with charity for the whole of creation, for men, for the birds, for the beasts, for the demons – for all creatures. He who has such a heart cannot see or call to mind a creature without his eyes becoming filled with tears by reason of the immense compassion which seizes his heart; a heart which is softened and can no longer bear to see or learn from others of any suffering, even the smallest pain, being inflicted upon a creature. This is why such a man never ceases to pray also for the animals.”
-St. Isaac the Syrian, cited in Vladimir Lossky’s, The Mystical Theology of the Eastern Church.