From The Meditations (second century CE), by Marcus Aurelius, translated in 1862 by George Long.

Men seek retreats for themselves, houses in the country, sea-shores, and mountains; and thou too art wont to desire such things very much. But this is altogether a mark of the most common sort of men, for it is in thy power whenever thou shalt choose to retire into thyself. For nowhere either with more quiet or more freedom from trouble does a man retire than into his own soul, particularly when he has within him such thoughts that by looking into them he is immediately in perfect tranquility; and I affirm that tranquility is nothing else than the good ordering of the mind. Constantly then give to thyself this retreat, and renew thyself; and let thy principles be brief and fundamental, which, as soon as thou shalt recur to them, will be sufficient to cleanse the soul completely, and to send thee back free from all discontent with the things to which thou returnest.

From The Private Papers of Henry Ryecroft (1912), by George Gissing.

Many a time, when life went hard with me, I have betaken myself to the Stoics, and not all in vain. Marcus Aurelius has often been one of my bedside books; I have read him in the night watches, when I could not sleep for misery, and when assuredly I could have read nothing else. He did not remove my burden; his proofs of the vanity of earthly troubles availed me nothing; but there was a soothing harmony in his thought which partly lulled my mind, and the mere wish that I could find strength to emulate that high example (though I knew that I never should) was in itself a safeguard against the baser impulses of wretchedness.

Hat tip: Graveyard Masonry.