Moral Letters to Lucilius Section 39

Et ut quod volo exprimam breviter, materia boni aliquando contra naturam est, bonum numquam, quoniam bonum sine ratione nullum est, sequitur autem ratio naturam. Quid est ergo ratio? Naturae imitatio. Quod est summum hominis bonum? Ex naturae voluntate se gerere.

Et pour résumer brièvement ma pensée, l’élément du bien est quelquefois contre nature, le bien ne l’est jamais, parce qu’il n’est aucun bien sans la raison et que la raison suit la nature. Qu’est-ce en effet que la raison ? L’imitation de la nature. Et le souverain bien ? Une conduite conforme au vœu de la nature.

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.