Full Idea
Socrates worries about the unity and teachability of virtue because he is torn between virtue as intellectual (unified and teachable) and virtue as natural (plural and unteachable).
Gist of Idea
Socrates is torn between intellectual virtue, which is united and teachable, and natural virtue, which isn't
Source
comment on Socrates (reports of career [c.420 BCE]) by PG - Db (ideas)
A Reaction
Admittedly virtue could be natural but still unified and teachable, but Socrates clearly had a dilemma, and this seems to make sense of it.