What’s the difference between a dark night of the soul, as understood by the Spanish Carmelite mystic, St. John of the Cross, and clinical depression? It is a topic that has intrigued me for over 15 years because my senior thesis as a religious studies major at St. Mary’s College was on St. John’s poem, “The Dark Night,” under the tutelage of Keith Egan, one of the country’s distinguished Carmelite scholars.

Back when Mother Teresa’s darker writings were published, I heard countless people, myself included, refer to it as depression. In response to a beautiful op-ed piece written by my friend, Fr. Jim Martin, in the New York Times, Deborah Leavy Haverford of Pennsylvania wrote “St. John of the Cross might have called the terrified sense of abandonment felt by Mother Teresa the ‘dark night,’ but nowadays it is known as depression.”

Um. Yes. And no.

Three years ago, Keith (my former college prof) mailed me a copy of his book, “Carmelite Prayer: A Tradition for the 21st Century.” I keep it beside my computer as a reminder of the wisdom and strength in those Carmelite saints that I love: St. John of the Cross, Teresa of Avila, and St. Therese of Lisieux. The eighth chapter of this compilation of essays by prominent Carmelite scholars is by Carmelite Kevin Culligan, O.C.D. (the Carmelite order, not obsessive-compulsive disorder), and is entitled “The Dark Night and Depression.”

Here are some excerpts from his chapter that help to distinguish the dark night of the spirit, which requires prayer, and clinical depression, which demands medical treatment.

In the dark night of spirit, there is a painful awareness of one’s own incompleteness and imperfection in relation to God, however, one seldom utters morbid statements of abnormal guilt, self-loathing, worthlessness, and suicidal ideation that accompany serious depressive episodes. Thoughts of death do indeed occur in the dark night of spirit, such as “death alone will free me from the pain of what I now see in myself” or “I long to die and be finished with life in this world so that I can be with God,” but there is not the obsession with suicide or the intention to destroy oneself that is typical of depression.

As a rule, the dark nights of sense and spirit do not, in themselves, involve eating and sleeping disturbances, weight fluctuations, and other physical symptoms (such as headaches, digestive disorders, and chronic pain).

I can usually tell whether persons are depressed or in the dark night by attending closely to my own interior reactions as these persons describe their inner experience. As a disorder of mood or affect, depression communicates across personal relationships. Depressed persons typically look depressed, sound depressed, and make you depressed. After listening to depressed persons describe their suffering, I myself begin to feel helpless and hopeless, as though the dejected mood of persons with depression is contagious. I also frequently feel deep pity for the “profound rejection and hatred of the self” that characterize persons who are truly depressed.

By contrast, I seldom feel down when I listen to persons describe the dryness of the dark night of sense of the painful awareness of God and self that accompany the dark nights of sense and spirit. Instead, I frequently feel compassion for what persons suffer as they are spiritually purified, together with admiration for their commitment to do all that God asks. In fact, at these times I feel my own self being energized. It seems that the strengthening of spirit that God brings to persons through darkness is also communicated to me.

With persons who symptoms are clearly only those of the dark night of sense or spirit, I assist them in the transition from meditative prayer to contemplative prayer in the case of dryness and by being a faithful and empathetic companion as they journey through the ups and downs of the dark night of the spirit. In these cases, I assume that, as John of the Cross observes, God’s self-communication is deepening in these persons, strengthening their spirit, transforming their customary ways of knowing and loving, and preparing them for divine union.

But we cannot always presuppose that persons are either in a dark night or in a serious depression. They may be experiencing both at the same time. A middle-aged father might concurrently be dry in his prayer and also struggling with feelings of worthlessness, losing sleep, and too preoccupied to enjoy downtime with his children or sexual relations with his wife due to a sudden and unexpected loss of employment that has placed his family’s financial security in serious jeopardy.

Or a woman in her fifties who feels the deep spiritual pain of not knowing whether God really exists may also discover that she cannot get over the grief–the morbid preoccupation with life having no meaning, psychomotor retardation, and suicidal ideation–she has felt since her husband’s death from brain cancer six months earlier. In these cases, I attend closely to the symptoms of the night, but do not discount the seriousness of the depression.

As a rule of thumb, if these symptoms continue for up to a month, I always recommend professional treatment for the depression. I point out the seriousness of their condition, the effectiveness of professional treatment with depression, and assure them that I will continue to be their spiritual guide, and in consultation with the other professionals involved in their care if that collaboration seems appropriate.

There are also times when persons do not manifest any of the characteristics of the critical periods of darkness in the journey of prayer, but may be seriously depressed as a result of such natural causes as genetic predisposition, biochemical imbalance, sociocultural influences, or an identifiable serious loss. When symptoms of serious depression emerge in persons I work with as a spiritual guide, I address them directly and, if they are serious enough, recommend professional treatment, often suggesting the family physician as the first resource.

Frequently, religious persons initially resist this treatment, regarding their symptoms as a sign of spiritual weakness and preferring to manage them with prayer and spiritual practices. I point out to them that their need for treatment itself may indeed be an occasion for spiritual growth, especially in self-knowledge and self-acceptance, and remind them of the known benefits of treatment, particularly medication, for stabilizing their emotions and allowing them to resume a regular prayer life.