Marcus Aurelius, Meditations
, Book 4 (tr. 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.