John Cassian, The Institutes IX.2: Of the care with which the malady of dejection must be healed

Wherefore if we are anxious to exert ourselves lawfully in the struggle of our spiritual combat we ought with no less care to set about healing this malady also. For “as the moth injures the garment, and the worm the wood, so dejection the heart of man.” With sufficient clearness and appropriateness has the Divine Spirit expressed the force of this dangerous and most injurious fault.