The Dark Night of the Soul by St. John of the Cross is a 16th-century mystical treatise that describes the soul's journey toward union with God through passive purification (spiritual cleansing). It describes the dark night phase as the suffering, confusion, and absence of God's presence necessary to let go of worldly attachments and the ego in order to achieve divine enlightenment. In modern psychology, the term is often associated with a crisis of faith or midlife crisis that leads to deeper self-growth. The work focuses on the transition from spiritual suffering to stable illumination (enlightenment).