386–429 · Augustine of Hippo, Letters 130.18
When we cherish uninterrupted desire along with the exercise of faith and hope and charity, we “pray always.” But at certain stated hours and seasons we also use words in prayer to God, that by these signs of things we may admonish ourselves, and may acquaint ourselves with the measure of progress which we have made in this desire, and may more warmly excite ourselves to obtain an increase of its strength. For the effect following upon prayer will be excellent in proportion to the fervour of the desire which precedes its utterance. And therefore, what else is intended by the words of the apostle: “Pray without ceasing,” than, “Desire without intermission, from Him who alone can give it, a happy life, which no life can be but that which is eternal”? This, therefore, let us desire continually from the Lord our God; and thus let us pray continually.