In his discourse “Of Tranquility” (2.2), Epictetus frames his general advice, that we should keep in mind what is up to us and what is not, using the analogy of someone about to litigate a dispute in court. He advises his listeners to “consider . . . what you wish to maintain and what you wish to succeed in.” It is always useful before entering the fray to have an ideal outcome in mind. Yet, in his typical fashion, Epictetus does not appear to be referring to the actual outcome of the dispute (which is external), but to that which is in the litigant’s control. What we should “wish to maintain,” according to him, is our freedom to choose to be rational and not overstep our bounds in our desires.
The key idea is that we are free to exercise our prohairesis, often translated as volition. From the entry on Epictetus in the Stanford Encyclopedia of Philosophy: “The volition [prohairesis], Epictetus argues, is ‘by nature unimpeded’ (1.17.21), and it is for this reason that freedom is for him an inalienable characteristic of the human being.” 
After Epictetus asks his listeners to consider what they wish to maintain or succeed in at court, he reminds them that “if you wish to maintain a will conformable to nature, you have every security, every facility, you have no troubles. For if you wish to maintain what is in your own power and is naturally free, and if you are content with these, what else do you care for?” Having “every security” is certainly desirable in this insecure world, but how easy is it to detach ourselves from outcomes? Stoicism is simple, but not easy–and yet the reward of otherwise-elusive tranquility is what spurs us on. Epictetus continues:
For who is the master of such things? Who can take them away? If you choose to be modest and faithful, who shall not allow you to be so? If you choose not to be restrained or compelled, who shall compel you to desire what you think that you ought not to desire? who shall compel you to avoid what you do not think fit to avoid?… The judge will determine against you something that appears formidable; but that you should also suffer in trying to avoid it, how can he do that?
Even when your fate is in the judge’s hands, you do have control over your thoughts, desires, and actions, which is a reassuring realization. It is a reminder that no set of circumstances has the power to erase your inner world, your very self, even though the future is unknown and beyond your control, and potentially more grim than the present. It takes a good deal of effort, however, to internalize Epictetus’ notion that no one else can make us suffer.
It is human and entirely normal to suffer amid hardships, but how long do we need to sink into this sense of suffering? Better to trick our mind into believing it is not suffering–or in Stoic terms, to refuse to assent to the impression that we should be suffering. Even if we are tricking ourselves, so what? Placebos have been shown to be as effective as medicine in countless studies. Suffer for a bit, but no longer than necessary.
Epictetus says that when Socrates was urged to prepare for his trial, he replied that he had been preparing for it his whole life, by learning what was right and what was wrong, and not doing anything unjust in his private or public life.  For the ancients, philosophy was far from mere pedantry or sophistry–it was a way, in fact the only way, to endure life’s most difficult situations (even, as in Socrates’ case, life-threatening ones).
Epictetus certainly had a way with words, I must say. I described him to someone I met at Stoicon in October as the Stoic I turn to when I need a kick in the ass. In this discourse, he goes on to say that if you should nonetheless cling to externals, then don’t hold back: be a slave, because that is what you are when you’ve given externals sway over your happiness.
For when you have subjected to externals what is your own, then be a slave and do not resist, and do not sometimes choose to be a slave, and sometimes not choose, but with all your mind be one or the other, either free or a slave, either instructed or uninstructed…
It appears that Epictetus felt that philosophical preparation would be useful in any situation, to help one maintain one’s volition, independence from externals, and immunity to suffering. He draws an analogy to someone who knows how to write, and can then form letters when any situation arises that requires literacy:
But if you have practised writing, you are also prepared to write (or to do) any thing that is required. If you are not, what can I now suggest? . . . if you gape after externals, you must of necessity ramble up and down in obedience to the will of your master.
Professor Gregory Sadler has put a presentation about prohairesis online , which notes that there are various translations of prohairesis, including “faculty of choice,” “moral purpose,” and “will.” To Sadler, a useful way to approach the topic is to focus on the Stoics’ distinction between what is in our power and what is not. I love his bullet-point list of things in our power, seemingly designed to penetrate the Stoicism-resistant brain (e.g., mine):
- Desires and aversions (and emotions)
- Choice and refusal (and what we do with duties)
- Assent, assumptions, opinion, judgements
- Preparation and purpose
- How we devote our attention (prosokhes)
- Extends to willing or choosing (thelein)
- Use (khresis) of externals and appearances
Prohairesis, or the capacity for choice, is what makes us human. It is reassuring to remember that despite outward forces that might compel us to do or refrain from doing things, it is ultimately our choice to pursue good, desire what is in our power, or disdain things outside our power to achieve.
I adore these ancient terms, especially the Greek ones. They each seem to contain a world of possibility within, and they make this quest for truth more fun, more like solving a mystery. How can we explain in modern English what the ancients meant by prohairesis? If that weren’t difficult enough, how do we embody it in our thoughts and actions?
 Graver, Margaret, “Epictetus”, The Stanford Encyclopedia of Philosophy (Fall 2016 Edition), Edward N. Zalta (ed.), at http://plato.stanford.edu/archives/fall2016/entries/epictetus/
 This report of Socrates’ words comes from Xenophon Memorabilia; I am working from the online version, with notes, of Epictetus’ Discourses here: http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.01.0236%3Atext%3Ddisc%3Abook%3D2%3Achapter%3D2