Letters 94.33

Again, it is written: “Cast out all false opinions concerning Good and Evil, but replace them with true opinions; then advice will have no function to perform.” Order in the soul can doubtless be established in this way; but these are not the only ways. For although we may infer by proofs just what Good and Evil are, nevertheless precepts have their proper rôle. Prudence and justice consist of certain duties; and duties are set in order by precepts.

Seneca, Letters to Lucilius 94.33, translated by Richard Mott Gummere