One of Epictetus main teachings, as well as that of other Stoics, was that most of the things which trouble us are outside of our control. But, since we don't recognize this to be the case, we often suffer trying to control things that we simply cannot.
Stoicism was one of the most popular and influential schools of philosophy in the Ancient World. One of the greatest of all Stoic philosophers was Epictetus (55-135 AD), a man who, despite being subjected to slavery, was one of the greatest and penetrating minds of his time.
Epictetus held that only through self-mastery could we live in accordance with nature. Self-mastery consists of the use of reason and living virtuously. Above all else, the philosophy of Epictetus was a practical one that sought to help people live a good and meaningful life. Epictetus believed that philosophy depended on self-knowledge. In his works, the Discourses and the Enchiridion, he emphasized that there are things that we can control—that which is in our power— and those we cannot—that which is not in our power. Self-knowledge means learning to discern just what those things are.
“Some things are in our control and others not. Things in our control are opinion, pursuit, desire, aversion, and, in a word, whatever are our own actions. Things not in our control are body, property, reputation, command, and, in one word, whatever are not our own actions.”
What is in our power, is the 'authority over ourselves' that we have regarding our capacity to judge what is good and what is evil. Outside our power are 'external things', which are 'indifferent' with respect to being good or evil. Roughly, they are things that 'just happen', and they are not in our power in the sense that we do not have absolute control to make them occur just as we wish, or to make them have exactly the outcomes that we desire. For example, sickness is not in our power because it is not wholly up to us whether we get sick, and how often, nor whether we will recover quickly or indeed at all. Now, it makes sense to visit a doctor when we feel ill, but the competence of the doctor is not in our power, and neither is the effectiveness of any treatment that we might be offered. So generally, it makes sense to manage our affairs carefully and responsibly, but the ultimate outcome of any affair is, actually, not in our power.
What is in our power is the capacity to adapt ourselves to all that comes about, to judge anything that is 'dispreferred' not as bad, but as indifferent and not strong enough to overwhelm our strength of character. That is, we have power over our own minds. The opinions we hold of things, the intentions we form, what we value and what we are averse to are all wholly up to us.
We have the ability to make meaningful choices and we are responsible for our actions. This enables us to master our mind and emotions and live a happy and fulfilled life. Indeed, an individual is free only to the extent that they cultivated self-control and live according to reason. Only by mastering our emotions and examining our thoughts can we take control of our lives. Trying to control things outside of our control is like flailing around in quicksand thinking we'll free ourself that way.At some level we all know there are just some things we can't control. What we're afraid of is losing the sense of control we think we have.
And, as the great Stoic of the Star Wars universe, Yoda, once put it, "The fear of loss is a path to the dark side."