What is important is that we bring forth prayer from out of the depths. The one who strives to pray will understand these things and distinguish between them: he will know when he speaks with his mouth, when with the heart—or with the spirit, because, in the end, it is not the heart which should speak, but the spirit in the heart. Little by little he becomes aware of these things, he distinguishes between them, he learns them. Just as I am able to recognize you because I have seen you so often, the same thing happens here. The lips move, or they may not move at all. What matters is that a cry should come forth from the depths which, like a loud roar, like an earthquake, will shake the heavens and finally force God to answer, and to say, ‘Why do you cry to me?’”
– ‘On Prayer’