Meditations 8.48

Remember that the ruling faculty is invincible, when self-collected it is satisfied with itself, if it does nothing which it does not choose to do, even if it resist from mere obstinacy. What then will it be when it forms a judgment about anything aided by reason and deliberately? Therefore the mind which is free from passions is a citadel, for man has nothing more secure to which he can fly for refuge and for the future be inexpugnable. He then who has not seen this is an ignorant man; but he who has seen it and does not fly to this refuge is unhappy.

Marcus Aurelius, Meditations 8.48, translated by George Long

Never forget that the ruling Reason shews itself unconquerable when, concentrated in itself, it is content with itself so it do nothing that it doth not will, even if it refuse from mere opposition and not from reason—much more, then, if it judge of a thing on reasonable grounds and advisedly. Therefore the Mind, unmastered by passions, is a very citadel, for a man has no fortress more impregnable wherein to find refuge and be untaken for ever. He indeed who hath not seen this is ignorant, but he that hath seen it and takes not refuge therein is luckless.

Marcus Aurelius, Meditations 8.48, translated by C. R. Haines