From Rohan Healey's Greeks to Geeks: Practical Stoicism in the 21st Century -
"Epictetus: 'Some things are in our control and others are 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.'
OK, so let's think about that for a second. On first reading it may sound counter-intuitive, and certainly it is not something that is generally taught . . ."
Healey then goes on to examine some of the ways in which some things that seem to be under our own control, such as our bodies, are in fact subject to outside controls and influences, such as disease, old age, or simple physical constraint.
"One of the main objectives of Stoic philosophy is first of all to identify what lies within our full control all of the time and what does not, and secondly to then only concern ourselves with what we do control, and show apathy to that which we do not. Just imagine for a moment that you no longer cared about or worried about that which is not within your control. Imagine not caring about what others do, think, or say; imagine not worrying about the weather, or the traffic, or your wealth or reputation. The vast majority of what makes up the stress, discomfort, and unhappiness in the average human is caused by people imagining that they have control of that which they don't. If all you have to worry about is your own opinions, beliefs, actions, and perceptions, imagine the peace and contentment you could achieve."
Of course, one of the great problems we face is that most of us feel we must invest emotionally in things outside our own control.