If you were going to buy 15 gallons of gasoline for your car, would you want to know that the gas pump that dispenses the gas into your car was accurate? You would want the scale to say it pumped 15 gallons in your car, when in fact it only pumped 14 gallons in your car. For this reason, laws have been established to ensure gas stations' pumps are checked on a regular basis to verify they are accurate. If they are not, then the gas station owner is notified and possibly fined for fraud.
All of this - the scales, the law, the verification, the fines - are set up to ensure justice between seller and buyer. What great care we take, as a society, over such small matters as gasoline. Yet what "scale" or system of measurement do we use to tell us if our judgements and way of living are accurate?
This is the point of philosophy - to tell us if our opinions and actions are appropriate.
We are born into "the world without having an innate conception of what is good and bad, right and wrong, appropriate and inappropriate, and of happiness, and of what is proper for us and falls to our log, and of what we ought to do and ought not to do" (v. 3, p. 93). As such, we default to the opinions of our parents and tribes. Repeated millions of times throughout the world, people and tribes arrive at different conclusions and opinions about what is good, bad, right, wrong and what actions are appropriate and not. If only there were some scale or measuring stick to tell us what is right from wrong, good from bad, appropriate versus inappropriate actions. But this is precisely what philosophy aims to do.
How can we tell which opinions are the correct opinion?
"Look now, this is the starting point of philosophy: the recognition that different people have conflicting opinions, the rejection of mere opinion so that it comes to be viewed with mistrust, and investigation of opinion to determine whether it is rightly held, and the discovery of a standard of judgement, comparable to the balance that we have devised for the determining of weights, or the carpenter's rule for determining whether things are straight or crooked" (v. 13, p. 94).
And to be absolutely clear, he further states, "the opinion that each person holds is not a sufficient criterion for determining the truth" (v. 14, p. 94).
We must devise a method and standard for ensuring our opinion is good. We, humans, have figured out a way to devise a standard for measuring and weighing things such as gasoline, gold, silver, the height of a basketball hoop, so "how is it possible that that which is most vital for human beings should lie beyond determination, beyond discovery?" (v. 16, p. 94)
There is a standard; and we must seek it out and discover it and then "put it to use without fail ever afterwards" (v. 17, p. 94). It will then "rescue [us] from madness" (v. 18, p. 94).
Epictetus then demonstrates two examples in the chapter by applying them to some criteria about what is good: 1) pleasure and 2) pride.
Since both are not constant and are unstable, they cannot be used as a measuring standard. For something to be good, it must be reliable and constant.
This is why the Stoics arrived at the conclusion that "virtue is the sole good" as virtue does not change.
"It is thus that things are judged and weighed when one has the standards at hand; and the task of philosophy lies in this, in examining and establishing those standards. As for the use of them, once they are known, that is the business of the virtuous and good person" (v. 23-25, p. 95).