Acedia was defined by Thomas Aquinas as the "lack of energy to begin new things." It is a kind of ennui, depression, cynicism, sadness, boredom, listlessness, couch-potato-itis, being passive, apathy, psychic exhaustion, having no energy. An ancient text from the monk Evagrius calls it the "noonday demon" that is "the most oppressive of all demons." The desert monks considered it the most dangerous of the demons, for it tempted a monk to flee from his vocation back to the comforts of "the world."

Matthew Fox, Sins of the Spirit, Blessings of the Flesh