This book is a 16th-century treatise to the poem The Dark Night of The Soul by the Spanish writer John of the Cross. It gives a commentary on the poem and practical advice on the ascetical life, the final aim which is the union with God. Part of the advice is based on the author's experience of the spiritual search. Together with the poem and the other treatise The Dark Night, this work belongs to the most important writings of the Spanish spiritual mysticism.