People are disturbed not by things, Epictetus says, but by their opinions about things. Between an event and your distress sits a judgement - this is terrible, I have been wronged, I cannot bear it - and that judgement is yours. It can be inspected before you agree to it.
The Stoics called the raw appearance an impression, and the act of signing off on it assent. Their practice lives in the gap: notice the impression, refuse to add to it, and see what is actually there when the commentary stops.
50 passages
"For if a person thinks a true composite judgement 22 to be false, the composite judgement does not suffer, but the person who has been deceived."
"'Everything is what you judge it to be.' While the retort made to the Cynic philosopher Monimus is plain enough, plain too is the use of the saying, if one only take the gist of it, so far as it is true."
"But every rational soul is by nature offended at contradiction, and so long as it does not understand this contradiction, it is not hindered from doing contradictory things: but when it does understand the contradiction, it must of necessity avoid the contradiction and avoid it as much as a man must dissent from the false when he sees that a thing is false; but so long as this falsehood does not appear to him, he assents to it as to truth."
"When a raven croaks inauspiciously, let not the external impression carry you away, but straightway draw a distinction in your own mind, and say, "None of these portents are for me, but either for my paltry body, or my paltry estate, or my paltry opinion, or my children, or my wife."
"This power is in a man's soul, if he is indifferent to what is indifferent; and he will be so, if he regard every one of these indifferent objects as a whole and in its parts, remembering that none of them creates in us a conception about itself nor even comes to us, but they are motionless, and it is we who create judgements about them and so to speak inscribe them on ourselves; and yet we need not inscribe them and, if we do so unconsciously, we can wipe them off again at once."
"Let your impulse be to see always and entirely what precisely it is which is creating an impression in your imagination, and to open it up by dividing it into cause, matter, relation, and into the period within which it will be bound to have ceased."
"The answer was “Yes.” Then I must use a demonstrative form of speech.—This was granted.—How then will you know if I am cheating you by my argument?"
"With everything which entertains you, is useful, or of which you are fond, remember to say to yourself, beginning with the very least things, "What is its nature?" If you are fond of a jug, say, "I am fond of a jug"; for when it is broken you will not be disturbed."
"Consider the causes of reality stripped of their covering; the relations of your actions; the nature of pain, pleasure, death, fame; who is not the author of his own unrest; how none is hindered by his neighbour; that all things are what we judge them to be."
"The pouring and diffusion of the understanding then should be similar, in no way a pouring out, but an extension, and it should not rest forcibly or violently on obstacles that meet it nor yet fall down, but stand still and illuminate the object that receives it; for that which does not reflect it will rob itself of the light."
"It is possible to entertain no thought about this, and not to be troubled in spirit; for things of themselves are not so constituted as to create our judgements upon them."
"Constantly stop and consider the manner of men these are whose testimony you desire to gain, and their ruling principles; for, if you look into the sources of their judgement and impulse, you will not blame those who stumble involuntarily nor will you invite their testimony to yourself."
"But when they are diffused into the understanding by dint of that other sympathy, as needs must be in a united system, then you must not try to resist the sensation, which is natural, yet the governing part must not of itself add to the affection the judgement that it is either good or bad."
"Let us believe nothing unless it forces itself upon our sight and is unmistakable, and let us reprove ourselves for being too ready to believe, as often as our suspicions prove to be groundless: for this discipline will render us habitually slow to believe what we hear."
"As if a man should stand by a sweet and crystal spring of water and curse it, but it never ceases bubbling up in water fresh to drink, and if he throw in mud or dung, it will quickly break it up and wash it away and will in no way be discoloured."
"Let what will from outside happen to what can be affected by this happening, for the parts which are affected shall, if they please, find fault; whereas I myself, unless I conceive the accident to be evil, am not yet harmed; and it is in my power not to conceive it to be evil."
"Let your impulse be to see always and entirely what precisely it is which is creating an impression in your imagination, and to open it up by dividing it into cause, matter, relation, and into the period within which it will be bound to have ceased."
"When you see someone weeping in sorrow, either because a child has gone on a journey, or because he has lost his property, beware that you be not carried away by the impression that the man is in the midst of external ills, but straightway keep before you this thought: "It is not what has happened that distresses this man (for it does not distress another), but his judgement about it." Do not, however, hesitate to sympathize with him so far as words go, and, if occasion offers, even to groan with him; but be careful not to groan also in the centre of your being."
"The happy man, therefore, is he who can make a right judgment in all things: he is happy who in his present circumstances, whatever they may be, is satisfied and on friendly terms with the conditions of his life."
"A third is in a sense not even conscious of what he has done, but he is like a vine which has borne grapes, and asks nothing more when once it has borne its appropriate fruit."
"The sphere of the soul is true to its own form, when it is neither extended in any way nor contracted inwards; when it is neither scattered nor dies down, but is lighted by the light whereby it sees the truth of all things and the truth within itself."
"Now there are two kinds of hardening, one of the understanding, the other of the sense of shame, when a man is resolved not to assent to what is manifest nor to desist from contradictions."
"We ought then in life also to have some skill as in the case of silver coin that a man may be able to say like the judge of silver, “Bring me any drachma and I will test it.” But in the case of syllogisms, I would say: Bring any man that you please, and I will distinguish for you the man who knows how to resolve syllogisms and the man who does not."
"You have ears, not merely that you may listen to musical sounds, softly and sweetly drawn out and harmonized: you ought to hear laughter and weeping, coaxing and quarrelling, joy and sorrow, the human voice and the roaring and barking of animals."
"I cannot deny that some do this out of sheer badness of nature; but more do so because lapse of time destroys their remembrance, for time gradually effaces what they felt vividly at the moment."
"Put before your eyes this ease with which reason will prove to be carried through all things (as fire moves upwards, a stone down, a roller on a slope) and ask for nothing more, for the remaining obstacles are either of the lifeless body or else do not overwhelm it or do any harm at all without the judgement and the consent of reason itself."
"Penetrate to the individuality of the cause and separating it from the matter, look into it; next isolate the time which at longest this individuality can by its nature subsist."
"Let not our ears be easily lent to calumnious talk: let us know and be on our guard against this fault of human nature, that we are willing to believe what we are unwilling to listen to, and that we become angry before we have formed our opinion."
"Make it, therefore, your study at the very outset to say to every harsh external impression, "You are an external impression and not at all what you appear to be." After that examine it and test it by these rules which you have, the first and most important of which is this: Whether the impression has to do with the things which are under our control, or with those which are not under our control; and, if it has to do with some one of the things not under our control, have ready to hand the answer, "It is nothing to me.""
"Wipe out impressions by continually saying to yourself: it is in my power now not to allow any wickedness to be in this soul of mine, any appetite or disturbance at all, but seeing what is the character of them all I employ each according to its worth."
This is the machinery underneath most of the rest of Stoicism. Anger, fear, grief and craving are all analysed as judgements, which is why the Stoics thought they could be argued with rather than merely endured. It is also the part of the tradition with the clearest modern descendant: the founders of cognitive behavioural therapy named Epictetus as a source.
What the passages do not offer is a technique. Marcus mostly just does it, over and over, on whatever is bothering him that day - which is probably the honest form of the instruction.