Plato solved a problem his teacher created. Socrates had spent his life asking 'what is justice?' or 'what is courage?' and demolishing every answer — but never offering one of his own. Plato, writing after Socrates' execution, set out to build the positive philosophy his master never wrote down. The result was a body of dialogues, with Socrates usually as the lead speaker, that range across nearly every question philosophy has since asked.
At the centre stands the Theory of Forms. Why can we recognise many different things as 'beautiful' or 'just' when no two are alike? Plato's answer: because each imperfectly reflects a single perfect Form — Beauty itself, Justice itself — existing in a realm beyond the senses, eternal and unchanging. The physical world is a shadow-play; genuine knowledge means turning away from shifting appearances toward these stable ideals, a journey he captured in the allegory of the cave.
The RepublicWho should rule?
Plato's most influential work, The Republic, uses the question of justice to design an ideal city. Its most provocative claim is that societies should be governed by philosopher-rulers — those who have grasped the Forms and so know the good itself, rather than those who merely chase power or popularity. The proposal is deeply controversial; later readers have called it visionary and authoritarian in equal measure. But it launched political philosophy as a discipline.
In 387 BCE Plato founded the Academy, a school of philosophy and mathematics that endured for centuries and gave us the word 'academic.' Among its students was a young Aristotle, who would absorb Plato's teaching and then turn against its core — rejecting the separate realm of Forms in favour of studying the world directly. That disagreement between teacher and student became one of the organising tensions of Western thought, and we are arguably still choosing sides.