"Supermen?" Lessing echoed. With an obvious effort he forced his mind into focus and sat up straighter, looking at Dyke with a thoughtful frowm. "Maybe. Or maybe—Lieutenant, do you ever read Cabell? In one of his books somewhere I think he has a character refer to a sort of super-race that impinges on ours with only one . . . one facet. He uses the analogy of geometry, and suggests that the other race might be represented by cubes that show up as squares on the plane geometric surface of our world, though in their own they have a cubic mass we never guess." He frowned more deeply and was silent.
What book are they talking about? There's not much to go on, but at least we know the author—James Branch Cabell—so this should be answerable.
The only books of Cabell's that I've read are Jurgen, Figures of Earth, The Silver Stallion, Something About Eve, and The Cream of the Jest. It's probably not one of those because the description rings no bells; however, it's been many years since I read them.