how can we tell which opinions are the correct opinion?
"Here you have philosophy's starting point: we find that people cannot agree among themselves, and we go in search of the source of their disagreement. In time, we come to scorn and dismiss simple opinion, and look for a way to determine if an opinion is right or wrong. At last, we focus on finding a standard that we can invoke, just as the scale was invented to measure weights, and the carpenter's rule devised to distinguish straight from crooked. That is the beginning of philosophy."
the idea here, is for us to be able to judge an opinion, we must measure it against something that does not change. Epictetus gives two examples in the chapter: 1) pleasure and 2) pride.
since both are not constant and can vary, they cannot be used as a measure for philosophy.
i suspect this is why the Stoics arrived at the conclusion that "virtue is the sole good" as virtue does not change.