The mind may become so intensely selfish that even the world cannot supply its wants. How many persons, the victims of intense avarice, of burning selfish sensuality, of overleaping ambition, have renounced and cursed the world, because even the world, with all its adaptedness to their desires, could not give all that they asked! Men of wealth, voluptuaries, statesmen, warriors, kings, worn out with indulgence, or disappointed in their boundless aspirations, have separated themselves from society, when probably it did not occur to them to separate from themselves. In forests and in dens of the earth, and wherever they could flee away, and shut themselves up alone, they have poured forth, not their prayers to God, but their misanthropy and hate against man. In leaving the world behind them, they have carried in their hearts that which gave the world its evil and its sin.
True spiritual solitude, in being something more than solitude of the body, and something more than solitude of the unholy mind, is solitude from that in the mind, whatever it may be, which tends to disunite and dissociate it from God.
The soul, in the state of interior solitude, is in a state of solitude or separation from two things, in particular, namely, from its own desires and its own thoughts. IT IS SEPARATE FROM ITS OWN DESIRES. Sick of the world, if thou wouldst erect an inward oratory, and enter into the secret place of the heart, then let it be thy first purpose, as it certainly is an indispensable one, to cease from all desire, except such as God himself animates. In order to control the desires, and bring them into subjection to God, it is necessary to control the senses. The desires must have their appropriate objects; and in a multitude of cases the objects are made known by the senses. Keep a close watch, therefore, upon the senses. Let not your eye rest upon anything which is forbidden. Let not your ear listen to any corrupting or unprofitable conversation; but be as one who has no sight, and no hearing, and no touch, and no taste for anything, except what God allows and is pleased with. Contend with all because all have gone astray. Crucify all, because all have crucified him, who is the Eternal Life. Separate from all, so far as they have separated from God; in order that being united with them in their truth, you may be united with the God of truth.
—edited from A Treatise on Divine Union (1851) Part 6, Chapter 10.