Plato was born in Athens of an aristocratic family. He recounts in the Seventh Letter, which, if genuine, is part of his autobiography, that the spectacle of the politics of his day brought him to the conclusion that only philosophers could be fit to rule. After the death of Socrates in 399, he travelled extensively. During this period he made his first trip to Sicily, with whose internal politics he became much entangled; sceptics about the authenticity of the Seventh Letter suppose it to be a forgery designed to support the opposition party of Dion against the tyrant Dionysius II. He visited Sicily at least three times in all and may have been richly subsidized by Dionysius. On return from Sicily he began formal teaching at what became the Academy. Details of Plato’s life are surprisingly sparse, partly because of the Athenian convention against naming contemporaries in literary works; Aristotle, for example, although a student at the Academy for some twenty years, gives us no information about Plato’s life. As a result the dating of his works has to be established on internal evidence, and is subject to scholarly dispute.

Plato’s fame rests on his Dialogues which are all preserved. They are usually divided into three periods, early, middle, and late. Early dialogues include Hippias Minor, Laches, Charmides, Ion, Protagoras, Euthyphro, Apology, Crito, Gorgias, Meno, Cratylus, and the doubtful Hippias Major, Lysis, Menexenus, and Euthydemus; middle dialogues include Phaedo, Philebus, Symposium, Republic, Theaetetus; and to the late period belong Critias, Parmenides, Phaedrus, Sophist, Statesman, Timaeus, and Laws. The early dialogues establish the figure of Socrates, portrayed as endlessly questioning, ruthlessly shattering the false claims to knowledge of his contemporaries. The aim of the elenctic method (see elenchus) is allegedly to clear the ground for establishing a just appreciation of virtue, but more is done negatively than positively. When Socrates asks ‘What is x?’ (virtue, justice, friendship, etc.), he is shown as brushing aside mere examples of x in favour of pursuit of the essence or form of x, or that which makes things x. In the middle dialogues, concern switches to the philosophical underpinnings of this notion of a form, possibly in response to pressure on Plato to justify the dialectical method as more than a sceptical game. The middle dialogues are not in dialogue form, and do not exhibit the Socratic method. The change may have been connected with Plato’s belief that the young should not be exposed to such drastic solvents (the teaching method of the Academy prohibited students younger than thirty years old from such exercises).

It is the middle dialogues that defend the doctrines commonly thought of as Platonism, and the positive doctrines are certainly uncompromising. A pivotal concept is that of the forms. These are independent, real, divine, invisible, and changeless; they share features of the things of which they are the form, but also cause them (so they are not simply common properties, or universals). Unique amongst them is the form of the good, the quasi-divine goal of mystical apprehension that could be achieved, if at all, only at the end of the philosophical pilgrimage.

Apprehension of the forms is knowledge (noēsis) whereas belief about the changing everyday world is at best opinion (doxa). Knowledge is recollection of the acquaintance we had with the forms before our immortal souls became imprisoned in our bodies (see anamnesis, beauty). The Republic develops the celebrated comparison between justice and order in the soul, and that in the state; the famous myth of the cave introduces the doctrine that only those who apprehend the form of the good are fit to rule.

The Parmenides and Theaetetus are late middle or early late dialogues, and the former contains sufficiently devastating criticism of the doctrine of forms to throw Plato’s later views into doubt. The latter is a brilliant investigation of the concept of knowledge that ushers in the classical and still widely accepted account of knowledge as true belief plus a logos, or certification by reason. In the late works, especially the last and longest dialogue, the Laws, Plato returns to the character of the ideal republic in a more sober manner, with civic piety and religion taking much of the burden of education away from philosophy. The Timaeus is especially interesting as a scientific treatise, whose cosmology echoed on in the Neoplatonism of the Christian era. Plato is generally regarded as the inventor of philosophical argument as we know it, and many would claim that the depth and range of his thought have never been surpassed.

http://www.ellopos.net/elpenor/greek-texts/ancient-greece/plato-homepage.asp A collection of online resources on Plato, including a biography (by W. K. C. Guthrie)