Two of the Thirty who ruled Athens by execution and confiscation, Critias and Charmides, were kin on the mother's side to a young aristocrat raised to expect a political career of his own. Sparta had installed that oligarchy after the city lost its long war in 404, and both men died fighting the democrats at Munychia, the hill above Athens's port, in 403. The democracy came back, and four years later it tried Socrates for impiety and killed him. Plato had watched his own class commit one crime and its enemies answer with another.
His response was not to withdraw from politics but to put a question in front of it: what is goodness, and can anyone actually know it? Everything else follows from taking that seriously: if justice is only what a city happens to agree on, then the Thirty and the jury that condemned Socrates were both simply doing politics, and there is no standing from which to object to either. So he argued that goodness, justice and beauty are real things, existing in their own right, unchanging, available to be known by thought rather than by looking. A chair in a room is a rough copy. What it copies is the truth.
That is the claim the Republic builds a city to test, and it is where the famous demand comes from: cities will not stop suffering until philosophers rule or rulers philosophise. It is easy to read as arrogance, but it is closer to a definition: power should be held by whoever has done the work of finding out what is actually good — and that person will not want it.
He built the institution the argument needed. Sometime in the mid-380s he began teaching in a grove sacred to the hero Hecademus, a mile northwest of the city walls, and the Academy that grew there taught mathematics as hard as it taught argument, on the view that only a mind trained on things that cannot be otherwise is fit for everything else. Aristotle arrived at seventeen, stayed twenty years, then rejected the theory of Forms outright, and the Academy went on.
The practical test came in Sicily and failed. Plato went to Syracuse three times, twice to work on the tyrant Dionysius II at the urging of his friend Dion, the tyrant's uncle and adviser at court; on the last visit, in 361, he was held against his will and got out through the intervention of Archytas of Tarentum, a Pythagorean mathematician who governed his own city. Dion invaded, took the city, and was assassinated by men he trusted. The Seventh Letter tells this story in the first person and reads it as proof of the thesis that cities will not be well governed until philosophers rule, which is one reason many scholars — Annas, Burnyeat, Irwin among them — doubt Plato wrote it at all.
The deepest attack on the Forms is Plato's own. In the Parmenides he puts a young Socrates in front of the old Parmenides, who had argued that nothing really changes, and lets the old man take the theory apart: if things are large by sharing in Largeness, then Largeness and the large things need a further Largeness to share in, and so on without end. Whether he ever answers it is still argued. He never speaks in a dialogue in his own voice either, which is why two and a half thousand years of readers have argued about what he thought.
The arguing is the afterlife: Byzantine manuscripts carried the text to the Renaissance, the Florentine Ficino's Latin translation was printed in 1484, and the page numbers scholars still cite come from the French printer Henri Estienne's edition of 1578.