Letters 71.32

The matter can be imparted quickly and in very few words: “Virtue is the only good; at any rate there is no good without virtue; and virtue itself is situated in our nobler part, that is, the rational part.” And what will this virtue be? A true and never-swerving judgment. For therefrom will spring all mental impulses, and by its agency every external appearance that stirs our impulses will be clarified.

Seneca, Letters to Lucilius 71.32, translated by Richard Mott Gummere