Letters 76.10

What then is peculiar to man? Reason. When this is right and has reached perfection, man’s felicity is complete. Hence, if everything is praiseworthy and has arrived at the end intended by its nature, when it has brought its peculiar good to perfection, and if man’s peculiar good is reason; then, if a man has brought his reason to perfection, he is praiseworthy and has reached the end suited to his nature. This perfect reason is called virtue, and is likewise that which is honourable.

Seneca, Letters to Lucilius 76.10, translated by Richard Mott Gummere