An important element of Yeats's understanding of the spiritual elements of the system, including the Principles and the Thirteenth Cone, came through his reading of Plotinus. Yeats read The Enneads as they came out in the translations of Stephen MacKenna, an Irish nationalist and great friend of John Millington Synge. Yeats had a copy of the first translation, Plotinus on the Beautiful (YL 1594, WBGYL 1606), which appeared from A. H. Bullen's Shakespeare Head Press in 1908, the same year as his own Collected Works. The Medici Society started to publish translations of the Enneads in order in 1917 (see YL 1589-93, WBGYL 1601-5)—the copy of Vol. 1 in the Yeats's library has George Yeats's bookplate (she already had copies of Thomas Taylor's early nineteenth-century selected translations.) The Yeatses also bought Dean Inge's Gifford Lectures on Plotinus in 1919 (YL 954, WBGYL 964).
By the time that A Vision A appeared, the first three volumes of MacKenna's translations, containing the first four Enneads, had been published. Yeats refers specifically to Plotinus's hypostases in "The Four Principles and Neo-Platonic Philosophy":
I have not considered the ultimate origin of things, nor have my documents thrown a direct light upon it. The word Anima Mundi frequently occurs and is used very much as in the philosophy of Plotinus. I am inclined to discover in the Celestial Body, the Spirit, the Passionate Body, and the Husk, emanations from or reﬂections from his One, his Intellectual Principle, his Soul of the World, and his Nature respectively. The Passionate Body is described as that which links one being to another, and that which rescues the Celestial Body from solitude, and this is part of the ofﬁce of the Soul of the World in Plotinus. As actually used in the documents Anima Mundi is the receptacle of emotional images when puriﬁed from whatever unites them to one man rather than to another. The 13th, 14th and 15th cycles are described as Spheres, and are certainly emanations from the Soul of the World, the Intellectual Principle and the One respectively, but there is a fundamental difference, though perhaps only of expression, between the system and that of Plotinus. In Plotinus the One is the Good, whereas in the system Good and Evil are eliminated before the Soul can be united to Reality, being that stream of phenomena that drowns us. (AVA 176, CW13 143-44)Yeats's schema is therefore relatively clear: the three supernatural cycles or Spheres are seen as expressions of Plotinus's three hypostases, The One, The Intellectual Principle, and The Soul of the World. There is a more tenuous relationship to the Four Principles, and this also presents the perennial problem of adapting a trinity to a quaternity or vice versa (one that he had encountered when writing on The Works of William Blake), but Yeats introduces a fourth term that probably owes as much to his work on Blake as reading of Plotinus: Nature. (In the WWB, Yeats had treated Nature as the mirror of the Holy Ghost, see YeatsVision.com.)
It is difficult from this simple set of correspondences to say how clear an idea Yeats actually had of Plotinus's hypostases or how far he saw the parallels as going. Part of the attraction of Neoplatonism in this context appears to be that it enabled Yeats to include elements of the automatic script that dealt with the Christian Trinity but within a form of pagan language. Plotinus's philosophy is eminently primary in most respects—"Plotinus' ecstasy" is after all the "ecstasy of the Saint" (AVA 215, CW13 177)—but his vision retains enough of antithetical Greek paganism to make it palatable to Yeats. The Trinity of Christian faith is given as the One (the Father), the Nous, Logos, or Intellectual Principle (the Son), and the Soul of the World (the Holy Ghost). While Yeats, writing as himself, identifies his Thirteenth, Fourteenth, and Fifteenth Cycles beyond physical incarnation with Plotinus’s hypostases (AVA 176, CW13 142–43), he has the character of Owen Aherne identify them with the Christian Trinity (AVA 236, CW13 194).
Importantly, though, Yeats continued his study of Plotinus as he rewrote A Vision. Stephen MacKenna wrote to his patron Ernest Debenham in October 1926 reporting his pleasure at an article by AE and added:
Another little encouragement: Yeats, a friend tells me, came to London, glided into a bookshop and dreamily asked for the new Plotinus, began to read there and then, and read on and on till he'd finished (he really has a colossal brain, you know), and now is preaching Plotinus to all his train of attendant Duchesses. He told my friend that he intended to give the winter in Dublin to Plotinus. (Journals and Letters of Stephen MacKenna, 235)What appears in the drafts of the late 1920s certainly shows increased attention given to Plotinus and his thought. (Continued in Part II.)