"The art of leather-working concerns itself with hides, but the art itself is altogether different from its material, the hides, and for that reason can't take itself as an object of examination."
In other words, the art of leather-work, works on, examines, and uses hides. The art of leather-work is not focused on, working on, or examining itself.
On the other hand, the art of reason and wisdom can and does work on, examine and use themselves, and their opposites. These things are unique to humans - this is "our nature" or how we were designed. This is essentially the work and object of philosophy.
"the most important task of a philosopher, and his first task, is to test out impressions and distinguish between them, and not to accept any impressions unless it has been duly tested" (v. 7, p. 47).
Epictetus talks about how people used to test coinage, to ensure it was real or fake and how they would have to develop a musician's ear to detect the slightest differences. He then says, "in matters where we think it makes a notable difference whether or not we go astray, we apply considerable attention to judging things that are liable to lead us astray; but when it comes to this poor ruling center of ours, we yawn and slumber, and accept any impression that comes along. For it doesn't occur to us that we'll suffer any damage as a result" (v. 10-11, p. 47).
This is a very important point he makes! We go to great lengths to ensure our coinage and dollar system is real and not fake. We've implemented an entire governmental system to ensure the integrity of our dollars. Yet, we barely make any effort to examine and confront impressions that come into our minds. Rather, we prefer to be tossed and thrown about by any event and impression that comes our way.
Someone cuts us off in traffic ... we fly into a rage! Someone breaks into our home and burglarizes us ... we feel sad, violated and insecure. We develop a heart condition ... we think "wo is me! my life is ruined; it's over!" and we pity ourselves. A manager has a bad day and takes it out on us ... we berate her behind her back and we feel upset. We find out we've been lied to by people we've trusted for so long ... we feel betrayed and bitter.
We fail to examine our impressions and we let them disturb us! But we don't have to! Events may happen and impressions may come, but we have the power of assent!
Someone cuts us off in traffic, we can think of reasons why maybe they are in a hurry, but there is no reason to be upset. Someone steals from you, then they've stolen from you. Do what you must to reclaim your possessions, but no reason to be upset. Take precautions to protect your home and be at peace. We develop a heart condition, but we already knew these things were out of our control, now we have to simply accept our fate and do what we must to ensure good health. A manager is upset with you, do what must be done to correct anything that needs to be corrected and give her the benefit of the doubt - she may simply be hungry! We've been lied to, no need to be upset, but learn who you can trust and cannot trust. Work with those who you can trust, don't work with those who you cannot trust; no need to feel bitter.
This is the most important art in life - the examination of impressions. This is learning and practicing the Stoic art of the Discipline of Assent. It "requires long preparation, and no end of effort and study" but "do you really expect to master the most important of the arts with little effort?"
We have to do the heavy mental lifting if we expect results.