This can be swiftly taught in very few words: virtue is the only good; there is no certain good without virtue; and virtue resides in our nobler part, which is the rational one. And what can this virtue be? True and steadfast judgment. For from this will arise every mental impulse, and by it every appearance that spurs our impulses will be rendered clear.

— Seneca, Moral Letters, 71.32 (resource) Ryan Holiday, The Daily Stoic