Plato lectured extensively at the Academy, and wrote on many philosophical issues, dealing especially in politics, ethics, metaphysics and epistemology. The most important writings of Plato are his dialogues, although a handful of epigrams also survived, and some letters have come down to us under his name.

It is believed that all of Plato's authentic dialogues survive. However, some dialogues ascribed to Plato by the Greeks are now considered by the consensus of scholars to be either suspect (e.g., First Alcibiades, Clitophon) or probably spurious (such as Demodocus, or the Second Alcibiades). The letters are all considered to probably be spurious, with the possible exception of the Seventh Letter.

The dialogues of Plato are lively, often humorous or ironic, full of memorable characters and humble detail. It is generally agreed that Plato is the most enjoyable of philosophers to read.

Socrates is often a character in the dialogues of Plato. How much of the content and argument of any given dialogue is Socrates' point of view, and how much of it is Plato's, is heavily disputed, since Socrates himself did not write down his teachings; this ambiguity is often referred to as the "Socratic problem". However, Plato was doubtless strongly influenced by Socrates' teachings, so many of the ideas presented, at least in his early works, were likely borrowings or adaptations.

Biography
Plato was born in Athens or Aegina in May or December in 428 or 427 BC. He was raised in a moderately well-to-do aristocratic family. His father was named Ariston, and his mother Perictione. His family claimed descent from the ancient Athenian kings, and he was related — though there is disagreement as to exactly how — to the prominent politician Critias. According to a late Hellenistic account by Diogenes Laertius, Plato's given name was Aristocles, whereas his wrestling coach, Ariston of Argos, dubbed him "Platon", meaning "broad" on account of his robust figure.

Diogenes mentions alternative accounts that Plato derived his name from the breadth (platutês) of his eloquence, or else because he was very wide (platus) across the forehead. According to Dicaearchus, Plato wrestled at the Isthmian games. Such was his learning and ability that the ancient Greeks declared him to be the son of Apollo and told how, in his infancy, bees had settled on his lips, as prophecy of the honeyed words which were to flow from them.

Plato became a pupil of Socrates in his youth, and—at least according to his own account—he attended his master's trial, though not his execution. He was deeply affected by the city's treatment of Socrates, and much of his early work records his memories of his teacher. It is suggested that much of his ethical writing is in pursuit of a society where similar injustices could not occur. During the twelve years following the death of Socrates, he traveled extensively in Italy, Sicily, Egypt, and Cyrene in a quest for knowledge.

After his return to Athens at the age of forty, Plato founded one of the earliest known organized schools in Western civilization on a plot of land in the Grove of Academe. The Academy was "a large enclosure of ground which was once the property of a citizen at Athens named Academus... some, however, say that it received its name from an ancient hero" (Robinson, Arch. Graec. I i 16), and it operated until 529, when it was closed by Justinian I of Byzantium, who saw it as a threat to the propagation of Christianity. Many intellectuals were schooled in the Academy, the most prominent one being Aristotle.

Plato was also deeply influenced by a number of prior philosophers, including: the Pythagoreans, whose notions of numerical harmony have clear echoes in Plato's notion of the Forms; Anaxagoras, who taught Socrates and who held that the mind, or reason, pervades everything; and Parmenides, who argued for the unity of all things and may have influenced Plato's concept of the soul.

Work

Themes
Unlike Socrates, Plato wrote down his philosophical views, leaving behind a considerable number of manuscripts.

In Plato's writings are debates concerning the best possible form of government, featuring adherents of aristocracy, democracy, monarchy, as well as other issues. A central theme is the conflict between nature and convention, concerning the role of heredity and the environment on human intelligence and personality long before the modern "nature versus nurture" debate began in the time of Thomas Hobbes and John Locke, with its modern continuation in such controversial works as The Mismeasure of Man and The Bell Curve.

Another key distinction and theme in the Platonic corpus is the dichotomy between knowledge and opinion, which foreshadow modern debates between David Hume and Immanuel Kant, and has been taken up by postmodernists and their opponents, more commonly as the distinction between the objective and the subjective. Even the story of the lost city or continent of Atlantis came to us as an illustrative story told by Plato in his Timaeus and Critias.

Plato also had a position on the art of writing as opposed to oral communication. This is evidenced in his Phaedrus1 dialogue and his Seventh Epistle.2 He said that oral communication is superior to the written word, especially in the accuracy of the oral word over the written word and in his Seventh Epistle that nothing of importance should be written down but transmitted orally.

Form and basis
Plato wrote mainly in the form known as dialogue. In the early dialogues, several characters discuss a topic by asking questions of one another. Socrates figures prominently, and a lively, more disorganised form of elenchos/dialectic is present; these are called the Socratic Dialogues.

The nature of these dialogues changed a great deal over the course of Plato's life. It is generally agreed that Plato's earlier works are more closely based on Socrates' thought, whereas his later writing increasingly breaks away from the views of his former teacher. In the middle dialogues, Socrates becomes a mouthpiece for Plato's own philosophy, and the question-and-answer style is more pro forma: the main figure represents Plato and the minor characters have little to say except "yes", "of course" and "very true". The late dialogues read more like treatises, and Socrates is often absent or quiet. It is assumed that while some of the early dialogues could be based on Socrates' actual conversations, the later dialogues were written entirely by Plato. The question of which, if any, of the dialogues are truly Socratic is known as the Socratic problem.

The ostensible mise en scène of a dialogue distances both Plato and a given reader from the philosophy being discussed; one can choose between at least two options of perception: either to participate in the dialogues, in the ideas being discussed, or choose to see the content as expressive of the personalities contained within the work.

The dialogue format also allows Plato to put unpopular opinions in the mouth of unsympathetic characters, such as Thrasymachus in The Republic.

Metaphysics
Platonism has traditionally been interpreted as a form of metaphysical dualism, sometimes referred to as Platonic or Exaggerated Realism. According to this reading, Plato's metaphysics divides the world into two distinct aspects: the intelligible world of "forms", and the perceptual world we see around us. The perceptual world consists of imperfect copies of the intelligible forms or ideas. These forms are unchangeable and perfect, and are only comprehensible by the use of the intellect or understanding — i.e., a capacity of the mind that does not include sense-perception or imagination.

This division can also be found in Zoroastrian philosophy, in which the dichotomy is referenced as the Minu (intelligence) and Giti (perceptual) worlds. The Zoroastrian ideal city, Shahrivar, also exhibits certain similarities with Plato's Republic. The existence and direction of influence here is uncertain; while Zoroaster lived well before Plato, few of the earliest writings of Zoroastrianism survive unaltered.

In the Republic Books VI and VII, Plato uses a number of metaphors to explain his metaphysical views: the metaphor of the sun, the well-known allegory of the cave, and most explicitly, the divided line.

Taken together, these metaphors convey a complex, and, in places, difficult theory: there is something called The Form of the Good (often interpreted as Plato's god), which is the ultimate object of knowledge and which, as it were, sheds light on all the other forms (i.e., universals: abstract kinds and attributes), and from which all other forms "emanate". The Form of the Good does this in somewhat the same way as the sun sheds light on, or makes visible and "generates" things, in the perceptual world.

In the perceptual world, the particular objects we see around us bear only a dim resemblance to the more ultimately real forms of Plato's intelligible world; it is as if we are seeing shadows of cut-out shapes on the walls of a cave, which are mere representations of the reality outside the cave, illuminated by the sun.

We can imagine everything in the universe represented on a line of increasing reality; it is divided once in the middle, and then once again in each of the resulting parts. The first division represents that between the intelligible and the perceptual worlds. This is followed by a corresponding division in each of these worlds: the segment representing the perceptual world is divided into segments representing "real things" on the one hand, and shadows, reflections, and representations on the other. Similarly, the segment representing the intelligible world is divided into segments representing first principles and most general forms, on the one hand, and more derivative, "reflected" forms, on the other.

Plato's metaphysics, and particularly its dualism between the intelligible and the perceptual, would inspire later Neoplatonist thinkers, such as Plotinus and Gnostics, and many other metaphysical realists. Although Platonist philosophers like Plotinus rejected Gnosticism (see Plotinus' Enneads). One reason being the Gnostic vilification of nature and Plato's demiurge from Timaeus. Plato also influenced Saint Justin Martyr. For more on Platonic realism in general, see Platonic realism and the Forms.

Although this interpretation of Plato's writings (particularly the Republic) has enjoyed immense popularity throughout the long history of Western philosophy, it is also possible to interpret his suggestions more conservatively, favoring a more epistemological than metaphysical reading of such famous metaphors as the Cave and the Divided Line. There are obvious parallels between the Cave allegory and the life of Plato's teacher Socrates (who was killed in his attempt to "open the eyes" of the Athenians). This example reveals the dramatic complexity that often lies under the surface of Plato's writing (remember that in the Republic, it is Socrates who relates the story.).

Epistemology
Plato also had some influential opinions on the nature of knowledge and learning which he propounded in the Meno, which began with the question of whether virtue can be taught, and proceeded to expound the concepts of recollection, learning as the discovery of pre-existing knowledge, and right opinion, opinions which are correct but have no clear justification.

Plato stated that knowledge is essentially justified true belief, an influential belief which informed future developments in epistemology. In the Theaetetus, Plato argued that belief is to be distinguished from knowledge on account of justification. Many years later, Edmund Gettier famously demonstrated the problems of the justified true belief account of knowledge.

The state
Plato's philosophical views had many societal implications, especially on the idea of an ideal state or government. There is some discrepancy between his early and later views. Some of the most famous doctrines are contained in the Republic during his middle period.

Plato asserts that societies have a tripartite class structure corresponding to the appetite/spirit/reason structure of the individual soul.

1. Productive (Workers) — the labourers, carpenters, plumbers, masons, merchants, farmers, ranchers, etc. These correspond to the "appetite" part of the soul.
2. Protective (Warriors) — those who are adventurous, strong, brave, in love with danger; in the armed forces. These correspond to the "spirit" part of the soul.
3. Governing (Rulers) — those who are intelligent, rational, self-controlled, in love with wisdom, well suited to make decisions for the community. These correspond to the "reason" part of the soul and are very few.

According to this model, the principles of Athenian democracy (as it existed in his day) are rejected as only a few are fit to rule. Instead of rhetoric and persuasion, Plato says reason and wisdom should govern. This does not equate to tyranny, despotism, or oligarchy, however. As Plato puts it:

"Until philosophers rule as kings or those who are now called kings and leading men genuinely and adequately philosophise, that is, until political power and philosophy entirely coincide, while the many natures who at present pursue either one exclusively are forcibly prevented from doing so, cities will have no rest from evils,... nor, I think, will the human race."

Plato describes these "philosopher kings" as "those who love the sight of truth" (Republic 475c) and supports the idea with the analogy of a captain and his ship or a doctor and his medicine. Sailing and health are not things that everyone is qualified to practice by nature. A large part of the Republic then addresses how the educational system should be set up to produce these philosopher kings.

However, it must be taken into account that the ideal city outlined in the Republic is qualified by Socrates as the ideal luxurious city, examined to determine how it is that injustice and justice grow in a city (Republic 372e). According to Socrates, the "true" and "healthy" city is instead the one first outlined in book II of the Republic, 369c-372d, containing farmers, craftsmen, merchants, and wage-earners, but lacking the guardian class of philosopher-kings as well as delicacies such as "perfumed oils, incense, prostitutes, and pastries", in addition to paintings, gold, ivory, couches, a multitude of occupations such as poets and hunters, and war.

Platonic scholarship
Plato's thought is often compared with that of his most famous student, Aristotle, whose reputation during the Western Middle Ages so completely eclipsed that of Plato that the Scholastic philosophers referred to Aristotle as "the Philosopher". However, in the Byzantine Empire, the study of Plato continued.

The Medieval scholastic philosophers did not have access to the works of Plato—nor the knowledge of Greek needed to read them. Plato's original writings were essentially lost to Western civilization until they were brought from Constantinople in the century before its fall, by George Gemistos Plethon. Medieval scholars knew of Plato only through translations into Latin from the translations into Arabic by Persian and Arab scholars. These scholars not only translated the texts of the ancients, but expanded them by writing extensive commentaries and interpretations on Plato's and Aristotle's works (see Al-Farabi, Avicenna, Averroes).

Only in the Renaissance, with the general resurgence of interest in classical civilization, did knowledge of Plato's philosophy become widespread again in the West. Many of the greatest early modern scientists and artists who broke with Scholasticism and fostered the flowering of the Renaissance, with the support of the Plato-inspired Lorenzo de Medici, saw Plato's philosophy as the basis for progress in the arts and sciences. By the 19th century, Plato's reputation was restored, and at least on par with Aristotle's.

Notable Western philosophers have continued to draw upon Plato's work since that time. Plato's influence has been especially strong in mathematics and the sciences. It inspired the greatest advances in logic since Aristotle, due to Gottlob Frege and his followers Kurt Gödel, Alonzo Church, and Alfred Tarski, the last of whom summarised his approach by reversing Aristotle's famous declaration of sedition from the Academy: Inimicus Plato, sed magis inimica falsitas ("Plato is an enemy, but falsehood is yet a greater enemy").

Albert Einstein drew on Plato's understanding of an immutable reality that underlies the flux of appearances for his objections to the probabilistic picture of the physical universe propounded by Niels Bohr in his interpretation of quantum mechanics. Conversely, thinkers that diverged from ontological models and moral ideals in their own philosophy, have tended to disparage Platonism from more or less informed perspectives. Thus Friedrich Nietzsche attacked Plato's moral and political theories, Martin Heidegger argued against Plato's alleged obfuscation of Being, and Karl Popper argued in The Open Society and Its Enemies (1945) that Plato's proposal for a government system in the Republic was prototypically totalitarian.

Bibliography
Plato's writings (most of them dialogues) have been published in several fashions; this has led to several conventions regarding the naming and referencing of Plato's texts.

By tetralogy
One tradition regarding the arrangement of Plato's texts is according to tetralogies. This scheme is ascribed by Diogenes Laertius to an ancient scholar and court astrologer to Tiberius named Thrasyllus.

In the list below, works by Plato are marked (1) if there is no consensus among scholars as to whether Plato is the author, and (2) if scholars generally agree that Plato is not the author of the work. Unmarked works are assumed to have been written by Plato.

Tetralogies
I. Euthyphro, (The) Apology (of Socrates), Crito, Phaedo
II. Cratylus, Theaetetus, Sophist, Statesman
III. Parmenides, Philebus, (The) Symposium, Phaedrus
IV. First Alcibiades (1), Second Alcibiades (2), Hipparchus (2), (The) (Rival) Lovers (2)
V. Theages (2), Charmides, Laches, Lysis
VI. Euthydemus, Protagoras, Gorgias, Meno
VII. (Greater) Hippias (major) (1), (Lesser) Hippias (minor), Ion, Menexenus
VIII. Clitophon (1), (The) Republic, Timaeus, Critias
IX. Minos (2), (The) Laws, Epinomis (2), Letters (1)

Works not in tetralogies
The remaining works were transmitted under Plato's name, most of them already considered spurious in antiquity:

Axiochus (2), Definitions (2), Demodocus (2), Epigrams, Eryxias (2), Halcyon (2), On Justice (2), On Virtue (2), Sisyphus (2)

Stephanus pagination
The usual system for making unique references to sections of the text by Plato derives from a 16th century edition of Plato's works by Henricus Stephanus. An overview of Plato's writings according to this system can be found in the Stephanus pagination article.

Chronology
The exact order in which Plato's dialogues were written is not known, nor is the extent to which some might have been later revised and rewritten. However, there is enough information internal to the dialogues to form a rough chronology. The dialogues are normally grouped into three fairly distinct periods, with a few of them considered transitional works, and some just difficult to place.

Many of the positions in this ordering are still highly disputed.

Early dialogues
Socrates figures in all of these, and they are considered the most faithful representations of the historical Socrates; hence they are also called the Socratic dialogues. Most of them consist of Socrates discussing a subject, often an ethical one (friendship, piety) with a friend or with someone presumed to be an expert on it. Through a series of questions he will show that they don't apparently understand it at all. This period also includes several pieces surrounding the trial and execution of Socrates.

1. Apology
2. Crito
3. Charmides
4. Laches
5. Lysis
6. Euthyphro
7. Menexenus
8. Lesser Hippias
9. Ion

The following are variously considered transitional or middle period dialogues:

1. Gorgias
2. Protagoras
3. Meno

Middle dialogues
Late in the early dialogues Plato's Socrates actually begins supplying answers to some of the questions he asks, or putting forth positive doctrines. This is generally seen as the first appearance of Plato's own views. The first of these, that goodness is wisdom and that no one does evil willingly, was perhaps Socrates' own view. What becomes most prominent in the middle dialogues is the idea that knowledge comes of grasping unchanging forms or essences, paired with the attempts to investigate such essences. The immortality of the soul, and specific doctrines about justice, truth, and beauty, begin appearing here. The Symposium and the Republic are considered the centrepieces of Plato's middle period.

1. Euthydemus
2. Cratylus
3. Phaedo
4. Phaedrus
5. Symposium
6. Republic
7. Theaetetus
8. Parmenides

Late dialogues
The Parmenides presents a series of criticisms of the theory of Forms which are widely taken to indicate Plato's abandonment of the doctrine. Some recent publications (e.g., Meinwald (1991)) have challenged this characterisation. In most of the remaining dialogues the theory is either absent or at least appears under a different guise in discussions about kinds or classes of things (the Timaeus may be an important, and hence controversially placed, exception).

Socrates is either absent or a minor figure in the discussion. An apparently new method for doing dialectic known as "collection and division" is also featured, most notably in the Sophist and Politicus, explicitly for the first time in the Phaedrus, and possibly in the Philebus. There is a question about the efficacy of the method for arriving at the answers to philosophical questions; it appears to rely for its results on little more than what its practitioners already believe or find intuitively plausible.

In that case it is not clear how it can bring about any progress on matters under discussion, and it has been suggested that the method is a sign of Plato's failing philosophical powers in his later life. It is far more likely, though, that scholars have simply yet to arrive at an adequate characterisation of the method.

A basic description of collection and division would go as follows: interlocutors attempt to discern the similarities and differences among things in order to get clear idea about what they in fact are. One understanding, suggested in some passages of the Sophist, is that this is what philosophy is always in the business of doing, and is doing even in the early dialogues.

The late dialogues are also an important place to look for Plato's mature thought on most of the issues dealt with in the earlier dialogues. There is much work still to be done by scholars on the working out of what these views are. The later works are agreed to be difficult and challenging pieces of philosophy. On the whole they are more sober and logical than earlier works, but may hold out the promise of steps towards a solution to problems which were systematically laid out in prior works.

1. Sophist
2. Statesman
3. Philebus
4. Timaeus
5. Critias
6. Laws