Yes, an old question, but I’m glad I ran into it so that I can think it through. I’m working from memory here, but didn’t Eliot do his Ph.D. thesis on the Idealist philosopher Bradley? That means he was strongly interested in Idealism, the view that the Ideas are the true reality, and that the world we live in contains copies or shadows of the Ideas. “Things below are copies…” as Yeats wrote. Or Shelley: “Life, like a dome of many-colored glass/Stains the bright radiance of Eternity/Till Death shatters it to fragments.”
I think that this notion is crossed with a Hindu/Buddhist/Zen notion of a void or gap:
So: There is a gap between the idea and the reality [It’s unbridgeable from an Idealist POV.]
Between the[beginning of the] movement and the completion—the finished act—there is a gap. I think that this is a gap of longing.
Between the conception and the creation; between the emotion and how we act in response to the emotion. Again a gap.
In the Japanese martial arts, the warrior is supposed to strike the opponent during this gap—when he has thought of attacking, but has not moved yet.
Between the desire and the spasm—hey, we’re talking about sex here. In between desire and orgasm is what Eliot thought of as the messiness of actual sexual activity. LOL. I think.
Potency/Existence: Potency is Aristotelian for potential. Existence is equivalent to actuality.
Essence/descent: Another Platonic (this time Neo-Platonic) formulation, The Essence is the Idea, which descends, step by step, until it becomes actual. Thus the Creator created leserbeings, who created lesser beings, which finally led us into this mess we call reality. The same image is part of the Medieval picture of the universe as represented by Dante: the earth is at the bottom of the universe, with Hall at its center. Soaring ove the earth are succfessively more excellent planetary spheres guided by angelic Intelligences. Farthest away, and surrounding all, is the sphere where God dwells, unimaginable to us, in the center of a celestial Rose.
All human activity, Eliot seems to be saying, (especially creative activity) is doomed to fall short. Later he described poetry (I’m not quoting this correctly) as ‘raids on the unsayable.’
Whew!