St. John Climacus
“Insensitivity is deadened feeling in body and spirit, and comes from long sickness and carelessness. Lack of awareness is negligence that has become habit. It is thought gone numb, an offspring of predisposition, a trap for zeal, a noose for courage, an ignorance of compunction, the gateway to despair, the mother of forgetfulness giving birth to loss of fear of God and, in turn, to a deadened spirit, like daughter bearing her own mother.”
Source: St. John Climacus, The Ladder of Divine Ascent