The Stoic concept of virtue suffers from being indefinable.
That’s not to say that definitions are not offered. They are. Some of them peel off some layers of meaning to make the concept a bit less fuzzy, such as that it means “moral excellence” and it can be subcategorized into the four cardinal virtues of practical wisdom, courage, justice, and temperance. These, in turn, can be further described, but none of that reaches the point.
The problem is that beyond that: The definitions are ultimately circular. They all depend on some undefinable idea of what is good. For example, Christopher Gill, writing on ModernStoicism.com gives the definition: “Virtue is a form or expertise or skill, knowledge how to live well in every way, a form of knowledge that shapes the whole personality and life.” This leaves out what that skill or knowledge happens to be, and what “live well” happens to mean. These are all vague terms.
Another definition of virtue as the fulfillment of our role as a rational and social animal is similarly vague. What is “our role”? How is a “rational and social animal” supposed to behave?
These definitions just kick the can down the road. When applied to specific situations, claims about what is and is not virtuous are widely disputed. Take a look at political discourse. There’s widespread disagreement about what is and is not virtuous. Perhaps the most glaring example of this is the abortion debate where most of the arguments are based directly on moral claims.
This is one of the biggest flaws in Stoicism. This criticism is not new. It dates back to the beginning of Stoicism. Stoicism’s founder, Zeno of Citium, surely heard his contemporary Timon of Phlius making this criticism (quoted by Sextus Empiricus in Against the Ethicists, section 140).
For the person who is in distress due to their avoidance of evil or pursuit of good, it will only be possible them to escape that distress by making it evident to them that there does not exist anything which is either good or evil by nature, ‘but these things are judged by mind on the part of humans,’ as Timon says. This teaching is unique to Pyrrhonism; and it therefore is Pyrrhonism’s achievement to secure eudaimonia.