Letters 118.13

People say: “You admit that that which is good is according to nature; for this is its peculiar quality. You admit, too, that there are other things according to nature, which, however, are not good. How then can the former be good, and the latter not? How can there be an alteration in the peculiar quality of a thing, when each has, in common with the other, the special attribute of being in accord with nature?”

Seneca, Letters to Lucilius 118.13, translated by Richard Mott Gummere