Only your own judgments and choices are truly within your control

Epictetus opens with what is arguably the single most influential idea in the text: things are divided into what is "up to us" — our opinions, aims, desires, and aversions — and what is "not up to us" — our body, property, reputation, and public office, along with the actions and opinions of other people.

He insists that the things within our control are naturally free and unhindered, while the things outside our control are inherently vulnerable to obstruction by circumstances, other people, or plain chance, however much effort we invest in trying to secure them.

Much of the text's practical advice flows directly from this division: because we cannot ultimately guarantee outcomes involving our health, our possessions, or how others treat us, investing our sense of identity and peace of mind in those outcomes guarantees periodic distress, while investing it in our own judgments and responses gives us something genuinely secure. Takeaway: ask, before reacting to anything, whether it is actually within your power to control — and if not, redirect your effort toward your response to it instead.