Zuckert, Catherine H., "Socrates' Positive Teaching," Plato's Philosophers: The Coherence of the Dialogues, Chicago: University of Chicago Press, 2009, 281-419.

Excerpt:

In the Apology Socrates says that, in response to the oracle’s paradoxical pronouncement that there was no one wiser than he, he went first to question the politicians because they claimed to know what is good. But in the first conversations Plato shows Socrates having after his “divine instruction” (by Diotima or the oracle), the philosopher found that neither his politically ambitious compatriots, nor the foreign sophists who claimed to be able to teach them, had a coherent understanding of the noble or good.

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