Plato finds it necessary to separate, for example, "horseness" from "horse" and say that horseness is real and fixed and true and unmoving, while the horse is a mere, unimportant, transitory phenomenon. Horseness is pure Idea. The horse that one sees is a collection of changing Appearances, a horse that can flux and move around all it wants to and even die on the spot without disturbing horseness, which is the Immortal Principle and can go on forever in the path of the Gods of old.

Ch. 29 - Zen and the Art of Motorcycle Maintenance (1974)