The oracle problem 

In Plato's Apology, Socrates explains to the court how his reputation began, and the explanation is a story about a religious consultation. His friend Chaerephon went to the oracle of Apollo at Delphi and asked whether anyone was wiser than Socrates. The priestess answered that no one was.

Socrates says he did not believe it and could not simply dismiss it either, since a god does not lie. So he treated the pronouncement as a riddle and set out to test it empirically, by finding someone wiser and thereby refuting the oracle. He went first to the politicians, then to the poets, then to the skilled craftsmen. The politicians were confident and could not explain themselves. The poets produced beautiful work and could not account for it, so that bystanders often spoke more sensibly about the poems than the poets did. The craftsmen genuinely knew their trades, which impressed him, but on the strength of that expertise they assumed they also understood the most important matters, and there they were as lost as everyone else.

His conclusion was narrow and easily overstated. He did not claim to know nothing. He claimed to know nothing about the greatest questions, how to live, what justice is, what the good is, and to be aware of that gap, whereas the men he interviewed had the same gap and did not know it. That is the whole of Socratic wisdom as the Apology presents it: not knowledge, but an accurate inventory of one's ignorance.

The narrative should be handled with some care. Xenophon also reports a Delphic pronouncement praising Socrates, though in different terms, which suggests the story circulated among his followers rather than being Plato's invention. Whether Chaerephon's visit happened as described is unrecoverable, and the account is doing obvious work in the speech, converting a lifetime of irritating interrogations into obedience to a divine instruction. That is exactly the defence a man charged with impiety would want.

It also explains, better than any other passage, why he was on trial at all. Socrates describes going systematically to the most respected men in Athens and demonstrating, in public, that their reputation for wisdom was unearned. He adds that the young men who followed him began imitating the technique, which multiplied the resentment and attached it to him. A method that produces humility in the practitioner produces humiliation in the subject, and the subjects in Athens were people with influence.

By 399 BC the resentment had found a legal form, and a jury of hundreds of citizens assembled to hear it.