A novel (1970) by John Gardner.

Agathon is an Athenian philosopher living (if you call it living, the way he does it) in Sparta 'round about the time of the Peloponnesian War, give or take a few years. Agathon is a social commentator and a drunk, an educated man on the skids, a former advisor to the Spartan dictator Lykourgos, an ongoing thorn in Lykourgos' side, and an advocate of rebellion by the helots, Sparta's serfs.

Agathon is a repulsive old man, but a funny one. He's the Greek philosopher for the rest of us(tm). He has an estranged wife and some kids back in Athens, and he turns out to have a grim history of mental illness, at times violent. Sometimes he thinks he's inspired by the gods; that might even be true, but then again his brain might just be turning backflips for the hell of it, like a cat on a leash.

The Wreckage of Agathon a mediation on power, government, civil disobedience and civic responsibility, madness, and guilt. The account of the Athenian ruler Solon is priceless. It's a good read. Really. Gardner was at his best with this kind of material. None of his contemporary novels ever grabbed me very much: He deals in grotesques and monsters, but he was too heavy-handed to put such things in upstate New York college towns and rural Vermont without turning them into silly caricatures. IMHO, of course.