First of all, it is worth pointing out that Plato’s moral education is not the mass education we have today, but an elite education. The purpose of moral education is to cultivate politicians and philosophers, that is, philosopher kings. This is the view put forward by Plato in his pinnacle work "The Republic".

Use a syllogism to illustrate:

Virtue is knowledge (major premise)

Knowledge is teachable (minor premise)

Therefore , Virtue can be taught (conclusion)

There is no problem with the minor premise. The key is to clarify the major premise: Is virtue a kind of knowledge? Regarding this issue, Socrates first pointed out that virtue is a good thing and a beneficial thing. Well, if something can be found that is beneficial but not knowledge, then virtue need not be knowledge. On the other hand, if no such thing is to be found, virtue can only be knowledge. Beneficial things, such as temperance, courage, justice, wealth, and good memory, are beneficial because we use them correctly and use them proportionately, that is, use them with wisdom. If we use them improperly, they can become harmful. So the beneficial thing must be some kind of wisdom. Therefore, virtue is a kind of knowledge, a kind of wisdom.