The book begins with an interesting passage: “Injustice is impiety. For since the universal nature has made rational animals for the sake of one another to help one another according to their deserts, but in no way to injure one another, he who transgresses her will is clearly guilty of impiety toward the highest divinity.”
Here the “divinity” can be interpreted, as usual, as god or nature, and Marcus is saying something remarkably modern: nature has made us (we would say has evolved us) into rational social beings, with what modern scientists may call a moral instinct, so to act unsocially or irrationally is literally going against (human) nature.
The idea that human beings ought not to behave irrationally is further developed a bit later (#2): “the destruction of the understanding is a pestilence, much more indeed than any such corruption and change of this atmosphere that surrounds us. For the latter corruption is a pestilence of animals so far as they are animals; but the former is a pestilence of men so far as they are men.”
At #4 Marcus tells us why it is irrational — according to the Stoic view of things — to do wrong: “He who does wrong does wrong against himself. He who acts unjustly acts unjustly to himself, because he makes himself bad.”
But what if others do wrong? We get the answer at #10: “If you are able, correct by teaching those who do wrong; but if you cannot, remember that indulgence is given to you for this purpose.”
And he returns to the idea of human beings as inherently social animals at #23: “As you yourself are a component part of a social system, so let every act of yours be a component part of social life.” Followed again by practical advise when we find out that people do or say bad things about us (#27): “When another blames you or hates you, or when men say anything injurious about you, approach their poor souls, penetrate within, and see what kind of men they are. You will discover that there is no reason to be concerned that these men have this or that opinion about you.”
Shortly thereafter (#28) we find one of Marcus’ recurring statements of “agnosticism”: “In a word, if there is a god, all is well; and if chance rules, do not also be governed by it.” I don’t think he was actually an agnostic, certainly not in the modern sense of the word, but this and many other quotes from the Meditations do attest pretty clearly that the Stoics didn’t think believing in god or providence was essential to arrive at their conception of ethics, which is the main reason modern practitioners are interested in Stoicism to begin with.
Not convinced? Here’s another one (#39): “Either all things proceed from one intelligent source and come together as in one body, and the part ought not to find fault with what is done for the benefit of the whole; or there are only atoms, and nothing else than mixture and dispersion. Why, then, are you disturbed?”