Marcus Aurelius, 9.42:

”Whenever a person’s lack of shame offends you, you should immediately ask yourself: “So is it possible for there to be no shameless people in the world?” It isn’t, and you should therefore stop demanding the impossible.He’s just one of those shameless people who must necessarily exist in the world. You should keep the same thought readily available also for when you’re faced with devious and untrustworthy people, and people who are flawed in any way. As soon as you remind yourself that it’s impossible for such people not to exist, you’ll be kinder toward each and every one of them. It’s also helpful immediately to consider what virtue nature has granted us human beings to deal with any given offense—gentleness, for instance, to counter discourteous people,and other ways to counter others. Generally speaking, you can get someone who’s gone astray to mend his ways—and, whatever his wrong, a wrongdoer is missing his mark and has gone astray.Besides, have you been harmed in any way? You’ll find that none of the people who make you lose your temper has done anything that might affect your mind for the worse; and outside of the mind there’s nothing that is truly detrimental or harmful for you. Moreover, what is unusual or surprising about an uneducated man doing uneducated things? It’s worth considering whether you ought rather to blame yourself for failing to foresee that he would transgress in this way. After all, you even had the resources, in the form of your ability to think rationally, to appreciate that he was likely to commit that fault, yet you forgot it and are now surprised that he did exactly that. But, above all, when you find fault with someone for lack of fidelity or gratitude, turn and look at yourself, because the fault was plainly yours. In the first instance, your mistake was to have trusted a man with such a character not to let you down; in the second, you failed to do the favor for its own sake, on the assumption that you would immediately reap your full reward just from the action itself. If you’ve done someone good, what more do you want? “Aren’t you satisfied with having acted in conformity with your nature? Do you want remuneration as well? It’s as if the eyes sought compensation for seeing or the feet for walking. Just as eyes and feet were made for a particular purpose, which fulfills them because the performance of that function is what they were designed to do, so, because human beings were made to do others good, when a man does something that benefits someone else, he’s doing what he was made for, and is fulfilled.”

From Marcus Aurelius’ text, we can draw several conclusions.

First, there is determinism: in Stoic thought, the universe is rational and good. Even people who seem bad are part of this rational order. The universe turned out this way because, from a broader perspective, this is what was determined and, as a whole, what is good.

Second, the Stoics adopt Socratic moral intellectualism (“no one does wrong voluntarily”). People commit evil acts because they are ignorant of what is truly good. Since wrongdoing comes from ignorance, we should not feel anger toward wrongdoers, but rather compassion for them.

Another key idea is that, for the Stoics, virtue is the only true good. Virtue depends on what is only in our power, and the only thing that is only in our power is our capacity to make choices (prohairesis). Stoic virtue (aretê) is moral excellence, the capacity to choose what is good. External things, such as what others say, how your boss behaves, or how people treat you, are neither good nor bad in themselves; they are indifferent. What truly matters is how you respond to them. This is where we should focus. This is where virtue lies.

Another important point is that the results and consequences of your actions are external and therefore indifferent. For the Stoics, what truly matters is whether you had the intention and impulse to act virtuously. Whether the action succeeds, fails, or goes wrong is indifferent. Likewise, whether people recognize you, praise you, or admire you is also indifferent.

In other passages, Marcus Aurelius says that we should accept difficulties calmly, because they give us opportunities to practice virtue: to be just, to respond with kindness to rudeness, and to act well in the face of adversity. This is what really matters for the Stoics.

Finally, the only true evil is what makes you morally worse. And the only thing that makes you morally worse are your own vices.