Theophan the Recluse was a Russian priest and bishop of the 19th century. In the latter half of his life he lived in solitude. Much of his life was spent writing and teaching about the life of prayer. In one of his homilies on prayer he offers the following:
Let me recall a wise custom of the ancient Holy Fathers: when greeting each other, they did not ask about health or anything else, but rather about prayer, saying “How is your prayer?” The activity of prayer was considered by them to a be a sign of the spiritual life, and they called it the breath of the spirit. If the body has breath, it lives; if breathing stops, life comes to an end. So it is with the spirit. If there is prayer, the soul lives; without prayer, there is no spiritual life.