Letters 58.18

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.

Seneca, Letters to Lucilius 58.18, translated by Richard Mott Gummere