Stoic Snippets 138

Do not look around you to discover other men's ruling principles, but look straight to this, to what nature leads you, both the Universal Nature through the things which happen to you, and your own nature through the acts which must be done by you. 

But every being ought to do that which is according to its constitution; and all other things have been constituted for the sake of rational beings, just as among irrational things the inferior for the sake of the superior, but the rational for the sake of one another. 

—Marcus Aurelius, Meditations 7.55