The dichotomy of control is Epictetus's division of everything into what is in our power and what is not. In our power: opinion, desire, aversion, and our own acts. Not in our power: the body, property, reputation and office — in a word, everything that is not our own act.
The paragraph the advice came from
Almost every modern account of Stoicism opens with some version of focus on what you can control. That advice is a paraphrase of one paragraph, and the paragraph is narrower than the paraphrase.
Of things some are in our power, and others are not. In our power are opinion (hupolaepsis), movement towards a thing (hormae), desire, aversion (echchlisis), turning from a thing; and in a word, whatever are our acts. Not in our power are the body, property, reputation, offices (magisterial power), and in a word, whatever are not our own acts. And the things in our power are by nature free, not subject to restraint or hindrance; but the things not in our power are weak, slavish, subject to restraint, in the power of others. Remember then, that if you think the things which are by nature slavish to be free, and the things which are in the power of others to be your own, you will be hindered, you will lament, you will be disturbed, you will blame both gods and men; but if you think that only which is your own to be your own, and if you think that what is another’s, as it really is, belongs to another, no man will ever compel you, no man will hinder you, you will never blame any man, you will accuse no man, you will do nothing involuntarily (against your will), no man will harm you, you will have no enemy, for you will not suffer any harm.
Notice how short the first list is. Opinion, movement towards a thing, desire, aversion — and then, gathering them, whatever are our acts. Everything in it happens inside a person. Nothing in it is an outcome.
That is the difficulty the paraphrase removes. Your effort is yours; the result of your effort is not. Your judgement about a person is yours; whether they stay is not. The line does not fall between big things and small, or between hard and easy. It falls between what you do and what happens.
The claim that rarely survives the summaries
Epictetus does not treat the confusion between the two lists as a mistake in reasoning. He treats it as the mechanism producing the whole catalogue of ordinary misery.
If you think the things in the power of others to be your own, he says, you will be hindered, you will lament, you will be disturbed, you will blame both gods and men. And if you do not: no man will compel you, no man will hinder you, you will accuse no man, you will have no enemy.
That inverts the usual direction of explanation. We normally say circumstances upset us. He is saying a category error upsets us, and circumstances only supply the occasion. Nothing in the argument requires the circumstances to be mild — Epictetus had been a slave, and the Discourses are full of prison, exile and death.
The cost he names before the benefit
If then you desire (aim at) such great things remember that you must not (attempt to) lay hold of them with a small effort; but you must leave alone some things entirely, and postpone others for the present. But if you wish for these things also (such great things), and power (office) and wealth, perhaps you will not gain even these very things (power and wealth) because you aim also at those former things (such great things); certainly you will fail in those things through which alone happiness and freedom are secured. Straightway then practise saying to every harsh appearance: You are an appearance, and in no manner what you appear to be. Then examine it by the rules which you possess, and by this first and chiefly, whether it relates to the things which are in our power or to things which are not in our power; and if it relates to anything which is not in our power, be ready to say that it does not concern you.
Leave alone some things entirely, and postpone others for the present. That is not a motivational line — it is a price, stated before anything encouraging arrives. The warning that follows is sharper still: aim at both and you may gain neither.
Then comes the only technique in the chapter, and it is a sentence to say out loud. To every harsh appearance: you are an appearance, and in no manner what you appear to be. Examine it, and ask first whether it belongs to the things in our power. If it does not, be ready to say that it does not concern you.
What desire and aversion mean in his list
Two of the four items in the first list — desire and aversion — get a chapter of their own, and it is the most technical thing in the handbook. Epictetus defines them by what they promise.
Remember that desire contains in it the profession (hope) of obtaining that which you desire; and the profession (hope) in aversion (turning from a thing) is that you will not fall into that which you attempt to avoid; and he who fails in his desire is unfortunate; and he who falls into that which he would avoid is unhappy. If then you attempt to avoid only the things contrary to nature which are within your power you will not be involved in any of the things which you would avoid. But if you attempt to avoid disease, or death, or poverty, you will be unhappy. Take away then aversion from all things which are not in our power, and transfer it to the things contrary to nature which are in our power. But destroy desire completely for the present. For if you desire anything which is not in our power, you must be unfortunate; but of the things in our power, and which it would be good to desire, nothing yet is before you. But employ only the power of moving towards an object and retiring from it; and these powers indeed only slightly and with exceptions and with remission.
Desire contains the profession of obtaining. Aversion contains the profession of not falling into. Both are claims about the future, and both can be broken by the world. He who fails in his desire is unfortunate; he who falls into what he would avoid is unhappy — two different failures, named separately.
The instruction that follows is the one modern summaries never repeat: for now, suppress desire entirely, and confine aversion to things within your own power. Not moderate your desires — remove them while you are learning. It is a training regime with a beginner's restriction, and it is why the Encheiridion reads as a manual rather than a philosophy.
Chapter 5, and the sentence everyone quotes
The dichotomy has a companion four chapters later, and it is the most quoted line in Stoicism.
Men are disturbed not by the things which happen, but by the opinions about the things; for example, death is nothing terrible, for if it were it would have seemed so to Socrates; for the opinion about death that it is terrible, is the terrible thing. When then we are impeded, or disturbed, or grieved, let us never blame others, but ourselves—that is, our opinions. It is the act of an ill-instructed man to blame others for his own bad condition; it is the act of one who has begun to be instructed, to lay the blame on himself; and of one whose instruction is completed, neither to blame another, nor himself.
Men are disturbed not by the things which happen, but by the opinions about the things. Note the example he reaches for — death is nothing terrible, for if it were it would have seemed so to Socrates. The argument is not that death is fine. It is that a thing which appears terrible to one person and not to another cannot have terribleness as a property.
Marcus Aurelius, saying it in one line
Epictetus taught; Marcus Aurelius wrote to himself, and the compression shows. The same doctrine appears in the Meditations as an instruction with the reasoning already assumed.
Take away thy opinion, and then there is taken away the complaint, "I have been harmed." Take away the complaint, "I have been harmed," and the harm is taken away.
Take away thy opinion, and then there is taken away the complaint. Take away the complaint, and the harm is taken away. Three clauses, each removing the ground under the next — and the last one is the claim that does the work: the harm was never in the event.
Do not have such an opinion of things as he has who does thee wrong, or such as he wishes thee to have, but look at them as they are in truth.
Do not have such an opinion of things as he has who does thee wrong, or such as he wishes thee to have, but look at them as they are in truth. That is the dichotomy applied to conflict: the other person's framing is not in your power, and adopting it is.
The failure mode
The dichotomy places effort; it does not prove that outcomes are unimportant. Read as permission to stop caring, it produces a detachment Epictetus never recommends — he is training people to act, and the whole handbook assumes they will.
The honest version is harder and duller: care about the outcome, work for it, and refuse to make your peace of mind depend on it. That is a discipline rather than a mood, which is why the book is a handbook and not an essay.
The Encheiridion is on this site in full — 52 chapters in George Long's translation, the same text the app narrates.