Letters 66.39

To explain my thought briefly, the material with which a good is concerned is sometimes contrary to nature, but a good itself never is contrary, since no good is without reason, and reason is in accordance with nature.

“What, then,” you ask, “is reason?” It is copying nature. “And what,” you say, “is the greatest good that man can possess?” It is to conduct oneself according to what nature wills.

Seneca, Letters to Lucilius 66.39, translated by Richard Mott Gummere