The single idea the rest of Stoicism is built on.
Before we get to advice about anger, fear, or grief, the Stoics start everyone in the same place: learning to tell the difference between what you can control and what you can't. Get this one idea into your bones and the rest of the philosophy clicks into place.
The clearest statement of it comes from Epictetus, a former enslaved man who became one of Rome's most influential teachers. He opens his handbook, the Enchiridion, with it:
"Some things are within our power, while others are not. Within our power are opinion, motivation, desire, aversion — in short, whatever is of our own doing. Not within our power are our body, our property, reputation, office — whatever is not of our own doing." — Epictetus, Enchiridion, §1
Up to you: your judgments, your choices, your effort, your attitude, how you respond. Not up to you: other people, the past, the weather, your reputation, and — crucially — outcomes.
The Stoic claim is bold but simple: nearly all of our anxiety comes from pouring energy into the second bucket. We try to control what other people think, whether we'll get the job, whether the plane will be on time — and we suffer when reality refuses to obey us. Peace doesn't come from controlling more. It comes from investing your energy only where you actually have power.
Let's put it to work.