Glaucon (Greek: Γλαύκων; c. 445 BC – 4th century BC) son of Ariston, was an ancient Athenian and the philosopher Plato's older brother. He is primarily known as a major conversant with Socrates in the Republic, and the interlocutor during the Allegory of the Cave. He is also referenced briefly in the beginnings of two dialogues of Plato, the Parmenides and Symposium.
Glaucon was the older brother of Plato, and like his brother was amongst the inner circle of Socrates’ young affluent students. Although little is known about his life, some information can be extrapolated from his brother’s writings and from later Platonic biographers.
His father was Ariston and his mother was Perictione. According to Diogenes Laërtius’ Life of Plato, Plato and Glaucon had a sister named Potone, and a brother named Adeimantus. In the dialogue Parmenides, a half-brother named Antiphon is also referenced.
According to the Oxford Greek Dictionary the name “Glaucon” is derived from the adjective glaukommatos (γλαυκόμματος) meaning “bright-eyed”, “owl-eyed”, or “grey-eyed”. This is generally considered to be a devotion to Athena, the goddess of wisdom and namesake and guardian deity of the city of Athens. It is not clear whether “Glaucon” was a name given at birth, an epithet for adoration of the goddess, or a nickname given for “looking for wisdom”. The use of epithets was not uncommon: for example, Plato’s birth name was Aristocles, but he was called the “wide” (platon) due either to his physical build or the breadth of his virtues.
Glaucon and at least one of his brothers fought against the Megarians in the Battle of Megara where the Athenians were defeated in 424 BC. This was during the height of the Peloponnesian War against Sparta and their allies. The brothers are commended for their “godlike” virtues in battle and for the strength of the bloodline by Socrates in the Republic.
It is not clear what Glaucon did for a living (if anything, as theirs was an aristocratic family). However, Socrates does say that Glaucon is a musician and thus can correctly answer questions about musical theory and harmonic proportion. This may also imply that, like many Athenians at the time including Plato himself, Glaucon studied the musical and mathematical theories of Pythagoras at some point.
Information on Glaucon’s life after the death of Socrates is unknown. As Plato’s dialogues of Socrates do not refer to Glaucon’s passing, he most likely died in or around Athens sometime after Socrates’ death in 399 BC.
Plato’s dialogues 
Glaucon is referenced briefly in the opening lines of this dialogue, along with his brother Adeimantus. They are visiting the agora of Athens, when they greet Cephalus, who is searching for their half-brother Antiphon because he supposedly has memorized the conversation between Socrates, Zeno, and Parmenides years before.
In the prologue of this dialogue, Apollodorus is speaking to Glaucon on the road to Athens about the drinking party (i.e. the Symposium), which occurred several years before in which Socrates and his fellows championed human and divine Love. Glaucon had heard a previous account, and the two talk about the event in order to “pass the time” on their way to Athens.
Plato's Republic begins with Socrates and Glaucon, who have just attended the inaugural Athenian celebration of the festival of Bendis, being playfully compelled by Polemarchus and Glaucon's brother Adeimantus and their companions to return with them to the house of Polemarchus, where they find Polemarchus' father Cephalus, his brothers Lysias and Euthydemus, and several other guests, including the sophist Thrasymachus. Socrates turns the conversation towards the definition of justice and refutes various accounts, in particular, that of Thrasymachus, who maintains that justice is "the advantage of the stronger"; Thrasymachus, that is, claims the authoritative element in each city makes the laws, and these are called "just".
Glaucon revives Thrasymachus' account and attempts to give it the strongest explication he can, because he wants to give Socrates a clear and forceful exposition of the claim that justice is valued only for its consequences and not in its own right. Glaucon explains that justice is a social contract that emerges between people who are roughly equal in power, such that no one is able to oppress the others, since the pain of suffering injustice outweighs the benefit of committing it.
No one, however, values justice for its own sake and everyone continues to look for opportunities to out-do his fellow citizens. In order to illustrate this point, Glaucon invokes the story of a ring of invisibility, which was found by an ancestor of Gyges, who then used his power to pursue his own advantage. Having told the story, Glaucon asserts that if there were two such rings, one given to a person who acts unjustly and the other to a person who acts justly, we would find that the just man, with his new-found power, begins to act exactly like the unjust man.
Glaucon is present for the remainder of the discourse, sharing duties as interlocutor with Adeimantus. In books 2-10, however, the interlocutors merely serve as philosophical foils to Socrates' exposition.
Glaucon is Socrates' interlocutor for various topics of discussion, such as the rearing and education of the just city’s “Guardian" class, the nature of beauty and ugliness, the qualities of the most evil type of man, and the subjects of thought in the immortal mind of Zeus.
Socrates questions Glaucon about animal husbandry, as related to the breeding of just individuals. It is mentioned that Glaucon is particularly knowledgeable in this topic as “you have in your house a number of hunting dogs and a number of pedigreed cocks.” Further along, Socrates mentions that Glaucon is a great lover of finery, and this leads to their conversation about the attributes and limitations of human love for beauty.
Elsewhere in Greek literature 
Glaucon appears in Xenophon's Memorabilia.  In the Memorabilia, Socrates seeks to save Glaucon, who was not yet twenty, from making a fool of himself before the ecclesia: he set out to make a speech and try and "preside" over the city, but Socrates reveals to Glaucon his utter ignorance of the actual affairs of state and convinces him not to speak. Glaucon, like many figures in the Memorabilia, is portrayed as rather dim-witted. The passage relating this tale is also notable because it includes the only direct reference in Xenophon's corpus to Plato, for whose sake Xenophon says Socrates intervened.
Glaucon appears in Aristotle's Poetics where Aristotle states: "The true mode of interpretation is the precise opposite of what Glaucon mentions. Critics, he says, jump at certain groundless conclusions; they pass adverse judgement and then proceed to reason on it; and, assuming that the poet has said whatever they happen to think, find fault if a thing is inconsistent with their own fancy."
- Diogenes Laërtius, iii. 3
- Diogenes Laërtius, iii. 4
- Pocket Oxford Classical Greek Dictionary, entry "γλαυκόμματος"
- Plato, Republic 368a.
- Plato, Republic, 398e
- Plato, Parmenides 126a-c
- Plato, Symposium 172b
- Plato, Republic 327a-328c
- Plato, Republic, 357a ff.
- Plato, Republic, 450a-b
- Plato, Republic, 506d
- Plato, Republic, 576b-c
- Plato, Republic, 608b-d
- Plato, Republic, 459a
- Plato, Republic, 474 c-e
- Xenophon, Memorabilia, Book III, chapter 6
- Aristotle, Poetics, 11.2
See also 
- List of speakers in Plato's dialogues
- Plato, Republic
- Plato, Symposium
- Plato, Parmenides
- Diogenes Laërtius, Life of Glaucon Translated by Robert Drew Hicks (1925).
- "Glaukon's Challenge" Glaucon's speech from Republic book 2. Translated by Cathal Woods (2010).
- Republic Translated by Paul Shorey (1935); annotated and hyperlinked text, English and Greek.
- Republic Translated by Benjamin Jowett (1892); with running comments and Stephanus numbers
- Republic Translated by Benjamin Jowett; with introduction