Glaucon (; Greek: ?; 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 in the inner circle of the young affluent students of Socrates. Although little is known about his life, some information can be extrapolated from Plato's writings and from later Platonic biographers.
His father was Ariston, and his mother was Perictione. According to Diogenes Laërtius, in his Life of Plato, Plato and Glaucon had a sister, Potone, and a brother, Adeimantus. In Plato's dialogue Parmenides, a half-brother, Antiphon, is also mentioned.
According to the Oxford Greek Dictionary the name "Glaucon" is derived from the adjective glaukommatos () meaning "bright-eyed", "owl-eyed", or "grey-eyed". It 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 unclear whether it 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 as a name was common: for example, Plato himself had the birth name Aristocles, according to Diogenes, but he was called "wide" (platon) for 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, with the Athenians victorious in 424 BC, 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 since his was an aristocratic family). However, Socrates says that Glaucon is a musician and so can correctly answer questions about musical theory and harmonic proportion, which may also imply that like many other Athenians at the time, such as Plato, Glaucon studied the musical and mathematical theories of Pythagoras at some point.
Not much is known about Glaucon's love life. In Republic, Socrates is quoted as saying to Glaucon: "I know you are or were in love with a lad like that, and I concede the point." It is unknown who this boy was. The context is a conversation about loving someone despite their having a physical flaw.
Information on Glaucon after the death of Socrates (399 BC) is unknown. As Plato's dialogues of Socrates do not refer to Glaucon's passing, he most likely died in or around Athens after Socrates.
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 memorised the conversation between Socrates, Zeno, and Parmenides, years before.
In the prologue of this dialogue, Apollodorus speaks to Glaucon on the road to Athens about a drinking party (symposium), which occurred several years before, when Socrates and his fellows championed human and divine Love. Glaucon had heard a previous account, and the two talk about the event 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 a 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 claims that the authoritative element in each city makes the laws, 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 so 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 outdo his fellow citizens. To illustrate the 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, that the just man, with his new 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 to 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 he has in his "house a number of hunting dogs and a number of pedigreed cocks". Later, Socrates mentions that Glaucon is a great lover of finery, which leads to their conversation about the attributes and limitations of human love for beauty.
Glaucon appears in Xenophon's Memorabilia.  There, Socrates seeks to save Glaucon, 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."