Letter 58 — On Being (§18)

“The third class is made up of those things which exist in the proper sense of the term; they are countless in number, but are situated beyond our sight. “What are these?” you ask. They are Plato’s own furniture, so to speak; he calls them “ideas,” and from them all visible things are created, and according to their pattern all things are fashioned. They are immortal, unchangeable, inviolable.”