The Imitation of Christ, by Thomas a Kempis, tr. by William Benham [1886], at sacred-texts.com
"My Son, precious is My grace, it suffereth not itself to be joined with outward things, nor with earthly consolations. Therefore thou oughtest to cast away all things which hinder grace, if thou longest to receive the inpouring thereof. Seek a secret place for thyself, love to dwell alone with thyself, desire the conversation of no one; but rather pour out thy devout prayer to God, that thou mayest possess a contrite mind and a pure conscience. Count the whole world as nought; seek to be alone with God before all outward things. For thou canst not be alone with Me, and at the same time be delighted with transitory things. Thou oughtest to be separated from thy acquaintances and dear friends, and keep thy mind free from all worldly comfort. So the blessed Apostle Peter beseecheth, that Christ's faithful ones bear themselves in this world as strangers and pilgrims.(1)
2. "Oh how great a confidence shall there be to the dying man whom no affection to anything detaineth in the world? But to have a heart so separated from all things, a sickly soul doth not yet comprehend, nor doth the carnal man know the liberty of the spiritual man. But if indeed he desire to be spiritually minded, he must renounce both those who are far off, and those who are near, and to beware of no man more than himself. If thou perfectly conquer thyself, very easily shalt thou subdue all things besides. Perfect victory is the triumph over oneself. For whoso keepeth himself in subjection, in such manner that the sensual affections obey the reason, and the reason in all things obeyeth Me, he truly is conqueror of himself, and lord of the world.
3. "If thou desire to climb to this height, thou oughtest to start bravely, and to lay the axe to the root, to the end that thou mayest pull up and destroy the hidden inordinate inclination towards thyself, and towards all selfish and earthly good. From this sin, that a man loveth himself too inordinately, almost everything hangeth which needeth to be utterly overcome: when that evil is conquered and put under foot, there shall be great peace and tranquillity continually. But because few strive earnestly to die perfectly to themselves, and do not heartily go forth from themselves, therefore do they remain entangled in themselves, and cannot be raised in spirit above themselves. But he who desireth to walk at liberty with Me, must of necessity mortify all his evil and inordinate affections, and must cling to no creature with selfish love."
(1) 1 Peter ii. 11.