Letters 65.13

Do you maintain that form is a cause? This is only what the artist stamps upon his work; it is part of a cause, but not the cause. Neither is the pattern a cause, but an indispensable tool of the cause. His pattern is as indispensable to the artist as the chisel or the file; without these, art can make no progress. But for all that, these things are neither parts of the art, nor causes of it.

Seneca, Letters to Lucilius 65.13, translated by Richard Mott Gummere