It is well-known that the Stoics claim that virtue is the only good. But there are four cardinal virtues, making up this “virtue”: courage, justice, moderation, and wisdom. This position is recognised in scholarly works. It also forms one organising basis for many practical works on how to live like a Stoic.
For practical purposes, it is usually enough to know that a good person on the Stoic model should be courageous in defending what is valuable to them, just to others, moderate in pursuing their desires, and wise overall in the choices they make, and the opinions they form about the world. The point is to try to be these things.
Nevertheless, there are questions here for the person looking to justify the practice. On one hand, defining what each of these four virtues involves, such that we can apply them wisely, is not easy. The state of the ancient sources which have survived — a great deal of Stoic texts have been lost — has almost certainly robbed us of material dealing with these subjects. In many cases, we have just one line descriptions of the different forms of moderation, courage, wisdom, justice.
On the other hand, why are there only four virtues in Stoicism? Well, “virtue” in the singular is presented as the only thing we should unconditionally value, as Stoics. The four virtues which make up this singular “virtue”, we have to assume, are somehow supposed to “cover everything” that might come to pass in our lives.
So, how or why could courage, with moderation, justice, and wisdom, set a person up to respond to any and all circumstances that might befall them, across the course of an adult life?
Here’s an answer. The Stoics recognise how much of what befalls a person in life is out of their sole control. Zeus alone, or nature, controls the whole. Other people have the same agency, and capacities to think, choose, speak and act which we do. We can’t control them either.
It follows that no person, even the most fortunate or powerful, will have a life in which nothing adverse or challenging ever happens to them — when their desires are thwarted, and when their wishes to avoid some person or thing cannot be fulfilled.