"The Importance of Distinguishing " from Rohan Healey's Greeks to Geeks: Practical Stoicism for the 21st Century
"Epictetus: Work, therefore, to be able to say to every harsh appearance, 'You are but an appearance, and not absolutely the thing you appear to be.' And then examine it by those rules which you have, and first, and chiefly, by this: whether it concerns the things which are in our own control, or those which are not; and, if it concerns anything not in our own control, be prepared to say that it is nothing to you.
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Epictetus is saying to us that if something is not within our power we have the right, or indeed the obligation, to say that it is nothing to us. If there is nothing we can do about something, what is the point in worrying? . . . You will find that in any circumstance you always have some power, to influence your opinion, and to make choices."
The ability to distinguish what is and is not in our power is perhaps the most important skill a Stoic learns, for everything hinges on the ability to make this critical distinction.