PLATO lived from about 427 to 347 BC in Athens.  He came from a prominent family and had all the advantages of money and connections.  Yet he became a follower of Socrates, a man from the working class who went around barefoot in a ragged cloak.  He tells us in his biographical Seventh Letter (a work whose authenticity has often been challenged, but which is very well-informed and now has the support of stylometric science behind it) that he considered joining the oligarchical government of the Thirty when the replaced the democracy in 404 BC.  He soon saw, as did everyone who stayed in Athens, that they were a vicious, self-serving junta.  Socrates refused to do their bidding and risked his life.  The Thirty were overthrown by a popular uprising.  

Socrates was later put to death for ostensibly religious reasons, but really for political reasons that motivated almost all trials in Athens.  Plato joined a number of the “Socratics,” the followers of Socrates, in publishing dialogues portraying Socrates as he was and vindicating him as a virtuous man seeking the good of Athens.  Plato was such a good writer that his dialogues became the literature of the fourth century. 

Plato made a voyage to southern Italy and Sicily when he was about forty years old, apparently to learn more about the Pythagorean philosophers who lived there.  He became friends with the leading Pythagorean philosopher and statesman Archytas, and also the powerful and ruthless tyrant of Syracuse, Dionysius and his brother-in-law Dion, who became a devoted follower of Plato.  Plato began to go beyond the ethical theories of Socrates, introducing the Pythagorean notions of immortality of soul (which Socrates was agnostic about) and reincarnation, as well as his own notion of ideal Forms such as the Forms of Justice and Equality, which gave their character to changeable things in this world.  

On returning to Athens, Plato founded his Academy, which provided a forum for intellectual from throughout the Greek world, and a kind of proto-university (but without degrees or formal courses of study).  Having defended the memory of Socrates in his early dialogues, he wrote his middle-period dialogues (see introduction to Socrates), in which he made Socrates a mouthpiece for his own theories of knowledge and reality (topics that Socrates did not study).  He also applied his theories to politics, developing the notion of an ideal state in his long dialogue The Republic, along with theories of education of the philosopher-kings he wanted to rule the state, psychology, theology, aesthetics, and other topics.

In his old age, he was invited back to Syracuse by his friend and disciple Dion, who hoped to instill philosophy in the new tyrant, Dionysius II, and create a more rational and perhaps ideal state.  But alas, the young tyrant sided with the enemies of Dion and had him exiled, leaving Plato in an awkward position of being dependent on an unstable dictator.  He finally was able to return home, only to be pressured to return a few years later to reconcile Dionysius II with Dion.  That project was another disaster, with the tyrant constantly manipulating the situation for his advantage.  Again, Plato was barely able to get away.  A few years later Dion led a revolt against the tyrant, which forced Dionysius II to flee.  But Dion was assassinated by a former member of Plato’s Academy in an embarrassing contretemps. 

Plato can be seen as the man who, more than any other, vindicated Socrates to the world.  Inspired by Socrates’ moral philosophy, he built a framework to buttress ethics through theories of knowledge (the Theory of Recollection), reality (the Theory of Forms), psychology (Immortality of Soul, Reincarnation, Tripartite Soul), politics (the Ideal State, different forms of government), and even natural science (Creationism, the centrifocal cosmos).  He provide perhaps the first comprehensive philosophical theory.