Epictetus is the best Stoic if you want to understand the fundamentals of Stoicism.
Pop Stoicism tells you that you should concern yourself only with the stuff that is under your control and leave out the rest. This makes enough sense. But it is not satisfying and giving enough for comprehension if you ask me.
What is under your control? How do you use your money? Which car to buy? Whether you say something or not?
It’s easy to believe, still after reading the phrase: “you should concern yourself purely with the things that are under your control” that this “control” that is being talked about, concerns things outside of yourself. Like the amount of money you save, or how you treat a person. But this would be incorrect, for the focus of “control” is still external.
In reality, the only thing that you can control completely is your reason. Epictetus explains it beautifully below:
In general, you will find no art or faculty that can analyze itself. Therefore, none that can approve or disapprove of itself. The art of grammar is restricted to the analyzing and commenting on literature. Music is confined to the analysis of harmony.
Consequently, neither of them analyzes itself.
Now, if you are writing to a friend, the art of grammar will help you decide what words to use, but it will not tell you whether it is a good idea to write to your friend in the first place.
Music is no different, whether this is a good time to sing and play, or a bad one. The art of music by itself cannot decide. So what can? The faculty that analyzes itself as well as others. Namely, the faculty of reason.
Reason is unique among the faculties assigned to us in being able to evaluate itself, what it is, what it is capable of, how valuable it is, in addition to passing judgment on others.
What decides whether a sum of money is good? The money is not going to tell you. It must be a faculty that makes use of such impressions. Reason.
Reason, in addition, takes the measure of music, grammar, and the other arts, judging their benefit, and deciding when it’s best to use them. — Epictetus