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. For with what art thou discontented? With the badness of men? Recall to thy mind this conclusion, that rational animals exist for one another, and that to endure is a part of justice, and that men do wrong involuntarily; and consider how many already, after mutual enmity, suspicion, hatred, and fighting, have been stretched dead, reduced to ashes; and be quiet at last.
Marcus Aurelius, Meditations. Book IV