Letters 58.19

And this “idea,” or rather, Plato’s conception of it, is as follows: “The ‘idea’ is the everlasting pattern of those things which are created by nature.” I shall explain this definition, in order to set the subject before you in a clearer light: Suppose that I wish to make a likeness of you; I possess in your own person the pattern of this picture, wherefrom my mind receives a certain outline, which it is to embody in its own handiwork. That outward appearance, then, which gives me instruction and guidance, this pattern for me to imitate, is the “idea.” Such patterns, therefore, nature possesses in infinite number,—of men, fish, trees, according to whose model everything that nature has to create is worked out.

Seneca, Letters to Lucilius 58.19, translated by Richard Mott Gummere