The Ascent of Mount Carmel is John of the Cross's systematic prose treatise on the same poem underlying the Dark Night of the Soul, but where the Dark Night describes what happens to the soul in the night's passive stages, the Ascent lays out the active discipline a person undertakes deliberately, above all detachment from sensory and intellectual attachments, summarized in John's famous repeated formula of nada, nothing, applied even to the summit of the mountain itself, since clinging to any created thing, however good, blocks the path to union with the uncreated God.
John structures the work in three books, moving through the night of the senses, then the active night of the spirit concerning the intellect, memory and will, and organizes his argument with the same careful, almost scholastic precision found throughout his prose writing, in deliberate contrast to the more purely poetic register of the underlying verse. John illustrates the discipline with the image of a bird that cannot fly while tied by even a single thread, however thin, arguing that a small remaining attachment blocks the soul's ascent as effectively as a heavy one, since what matters is the fact of attachment rather than its size.