Letters 76.14

You will speak of a sword as good, not when its sword-belt is of gold, or its scabbard studded with gems, but when its edge is fine for cutting and its point will pierce any armour. Take the carpenter’s rule: we do not ask how beautiful it is, but how straight it is. Each thing is praised in regard to that attribute which is taken as its standard, in regard to that which is its peculiar quality.

Seneca, Letters to Lucilius 76.14, translated by Richard Mott Gummere