Letters 58.21

Do you ask me where the difference lies? The former is the pattern; while the latter is the shape taken from the pattern and embodied in the work. Our artist follows the one, but the other he creates. A statue has a certain external appearance; this external appearance of the statue is the “form.” And the pattern itself has a certain external appearance, by gazing upon which the sculptor has fashioned his statue; this is the “idea.” If you desire a further distinction, I will say that the “form” is in the artist’s work, the “idea” outside his work, and not only outside it, but prior to it.

Seneca, Letters to Lucilius 58.21, translated by Richard Mott Gummere