On Pure and Sincere Prayer:

And therefore in order that prayer may be offered up with that earnestness and purity with which it ought to be, we must by all means observe these rules. First all anxiety about carnal things must be entirely got rid of; next we must leave no room for not merely the care but even the recollection of any business affairs, and in like manner also must lay aside all backbitings, vain and incessant chattering, and buffoonery; anger above all and disturbing moroseness must be entirely destroyed, and the deadly taint of carnal lust and covetousness be torn up by the roots. And so when these and such like faults which are also visible to the eyes of men, are entirely removed and cut off, and when such a purification and cleansing, as we spoke of, has first taken place, which is brought about by pure simplicity and innocence, then first there must be laid the secure foundations of a deep humility, which may be able to support a tower that shall reach the sky; and next the spiritual structure of the virtues must be built up upon them, and the soul kept free from all conversation and from roving thoughts that thus it may by little and little begin to rise to the contemplation of God and to spiritual insight. For whatever our mind has been thinking of before the hour of prayer, is sure to occur to us while we are praying through the activity of the memory. Wherefore what we want to find ourselves like while we are praying, that we ought to prepare ourselves to be before the time for prayer. For the mind in prayer is formed by its previous condition, and when we are applying ourselves to prayer the images of the same actions and words and thoughts will dance before our eyes, and make us either angry, as in Our previous condition, or gloomy, or recall our former lust and business, or make us shake with foolish laughter (which I am ashamed to speak of) at some silly joke, or smile at some action, or fly back to our previous conversation. And therefore if we do not want anything to haunt us while we are praying, we should be careful before our prayer, to exclude it from the shrine of our heart, that we may thus fulfill the Apostle's injunction: "Pray without ceasing;" and: "In every place lifting up holy hands without wrath or disputing." For otherwise we shall not be able to carry out that charge unless our mind, purified from all stains of sin, and given over to virtue as to its natural good, feed on the continual contemplation of Almighty God.

SAINT JOHN CASSIAN'S CONFERENCES, THE FIRST CONFERENCE OF ABBOT ISAAC ON PRAYER, CHAPTER III.