Quotes from Roman emperor Marcus Aurelius:
Nowhere can man find a quieter or more untroubled retreat than in his own soul.
To live happily is an inward power of the soul.
If thou workest at that which is before thee, following right reason seriously, vigorously, calmly, without allowing anything else to distract thee, but keeping thy divine part pure, as if thou shouldst be bound to give it back immediately; if thou holdest to this, expecting nothing, fearing nothing, but satisfied with thy present activity according to Nature, and with heroic truth in every word and sound which thou utterest, thou wilt live happy. And there is no man who is able to prevent this.