The “Spiritus Mundi” is, according to the speaker in “The Second Coming,” the source of the bizarre vision involving the sphinx with its slowly moving thighs, angry birds, and apocalyptic mood. But what did Yeats mean by this “spiritus mundi,” this world-soul from which visions somehow appear?
To understand what Yeats means by this, the best place to look might actually be in the works of another famous 20th-century thinker—Carl Jung—a psychologist who shared Yeats’s interest in the occult and specifically in the idea of a collective consciousness, which he called an oversoul. Jung took the phrase "over-soul" from an essay by Ralph Waldo Emerson, called "The Over-Soul," a winding and poetic piece of work in which he described the oversoul as an entity "within which every man's particular being is contained and made one with all other; that common heart."
According to Jung, the collective unconscious/oversoul contains archetypal symbols, such as the Great Mother, the Shadow, the Tower, the Tree of Life, Water, and more. It is the source of archetypal images that appear in dreams and give clues to the contents of the unconscious mind and to the threads that bind all human experience together. Jung believed that the world itself has a sort of “unconscious mind.” This collective consciousness or oversoul is similar to Yeats’s idea of the “Spiritus Mundi."
One aspect of this universal mind, in Jung's scholarship, was the concept of a universal memory, a collective recollection that all people share. Yeats also believed that we all inherited a common memory. In an essay called Philosophy of Shelley's Poetry, written in 1900, Yeats wrote, "Nor I think has any one, who has known that experience with any constancy, failed to find some day in some old book, or on some old monument, a strange or intricate image, that had floated up before him, and grown perhaps dizzy with the sudden conviction that our little memories are but a part of some great memory that renews the world and men's thoughts age after age, and that our thoughts are not, as we suppose, but a little foam upon the deep." If you've ever had a moment where you remember something that you are sure you have seen before, Yeats is saying, then you've felt your connection to the wider oversoul.
Yeats believed that history operates in cycles, and so images of the apocalypse arising out of the "Spiritus Mundi" may actually be memories of a previous apocalypse surging back up to the surface of the speaker's mind. Perhaps this is "The Second Coming": a flashback to memories of an apocalypse, forgotten by the self, but remembered by the oversoul.
Regardless, Yeats, like Jung, believed that we are all connected to forces much larger than us; we all share in the same memories, and we all know the same sacred symbols. But where Jung was more interested in psychoanalysis, Yeats was more interested in the ontological and poetic implications of these ideas—and out of these interests rose "The Second Coming."