This is the first line of the Enchiridion and the sentence most people meet Stoicism through. It is narrower than it sounds: the Stoics did not say outcomes do not matter, they said outcomes are not yours, and that the assent you give to an impression is. Epictetus taught it as a sorting exercise done on the spot, dozens of times a day, not as a doctrine to agree with. Its modern career runs through Albert Ellis and cognitive behavioural therapy, whose founders cited Epictetus directly, which is why the revival arrived with psychotherapists in it.