Plato's Dialogues: Meno
Plato’s dialogue Meno begins with the titular character asking Socrates whether virtue is something that can be taught. Socrates, of course, wants to begin by defining what exactly virtue is. Now, in Lysis, Laches, and Charmides, Socrates and friends couldn’t even figure out what a few particular virtues are, so it seems unlikely that we’ll find out what virtue as a whole is (spoiler: we don’t), but interestingly, unlike those three aporetic dialogues, Socrates does present a positive argument of his own and even offers a conclusion at the end.