What are the core ideas of Stoicism?

Start with the distinction Epictetus puts first in the Handbook: some things are up to us and some things are not. Opinion, pursuit, desire, aversion, in a word whatever is our own doing, is up to us. Body, property, reputation, command, in a word whatever is not our own doing, is not (Handbook 1). This is not a claim about willpower or influence. It is a claim about what actually belongs to you. Your body can be injured, your property taken, your reputation ruined by people who never meet you. None of that is fully yours to secure. Your judgments, your reaching for or away from things, your choice to act: nothing external can reach in and take those from you.

The common misreading treats this as a stress management trick: focus on what you can control and let go of the rest, and you will feel calmer. Epictetus is making a stronger claim than that. He says that if you mistake what belongs to others for your own, you will be hindered, you will lament, you will find fault with gods and men (Handbook 1). If you keep the distinction straight, no one will ever compel or restrain you, you will find fault with no one, you will do nothing against your will (Handbook 1). The payoff is not comfort. It is freedom, in the specific sense of never being forced into a false judgment by circumstance. That freedom is worth wanting only if you also accept the Stoic claim about value: happiness and freedom come only from the things that are up to us, and if you chase power or riches alongside them, you risk losing the very things that make you happy (Handbook 1).

The second core idea explains why the world so often looks like it is doing the disturbing. Epictetus states it plainly: men are disturbed not by things but by the judgments they form about things. Death itself is not terrible, or it would have appeared terrible to Socrates too. The terror is in our opinion that death is terrible (Handbook 5). This is the cognitive core of Stoic psychology. An emotion like grief or fear is not a direct reaction to an event. Epictetus even sorts people by how they handle this. Someone with no training blames others for their own bad condition. Someone starting out blames himself. Someone fully trained blames neither (Handbook 5). That last stage is not indifference. It is seeing clearly enough that blame, in either direction, no longer looks like the right response to a disturbance rooted in your own judgment.

Underneath both ideas is a claim about what makes a person capable of any of this in the first place: the rational faculty. Epictetus points out that no other capacity examines itself. Grammar can tell you which words are correct but not whether to write at all. Music can judge melody but not whether you should be singing right now. Only reason turns on itself, tests its own workings, and judges appearances as appearances (Discourses 1.1). That is the faculty the dichotomy of control and the theory of judgment both depend on: your capacity to examine an impression before you assent to it, rather than being carried along by it.

Seneca adds the piece that ties these ideas into a life rather than a set of separate techniques. He tells Lucilius that whenever you want to know what to seek or avoid, you should measure it against the Supreme Good, the purpose of your whole life, because no one can arrange the details of life without first fixing its chief aim (Moral Letters 71.2). His diagnosis for most people's mistakes is that we consider the parts of life but never life as a whole (Moral Letters 71.2). Put together, these are not separate slogans. They are one picture: reason lets you examine your own judgments, the dichotomy of control tells you what those judgments should be about, and a single purpose gives you the standard for judging well, letter by letter, choice by choice, across an entire life.