Plato (/ˈpleɪtoʊ/ PLAY-toe; Greek: Πλάτων, Plátōn; born c. 428–423 BC, died 348/347 BC) was an ancient Greek philosopher of the Classical period who is considered a foundational thinker in Western philosophy and an innovator of the written dialogue and dialectic forms. He influenced all the major areas of theoretical philosophy and practical philosophy, and was the founder of the Platonic Academy, a philosophical school in Athens where Plato taught the doctrines that would later become known as Platonism.

Plato's most famous contribution is the theory of forms (or ideas), which aims to solve what is now known as the problem of universals. He was influenced by the pre-Socratic thinkers Pythagoras, Heraclitus, and Parmenides, although much of what is known about them is derived from Plato himself.

Along with his teacher Socrates, and his student Aristotle, Plato is a central figure in the history of Western philosophy. Plato's complete works are believed to have survived for over 2,400 years—unlike that of nearly all of his contemporaries. Although their popularity has fluctuated, they have consistently been read and studied through the ages. Through Neoplatonism, he also influenced both Christian and Islamic philosophy. In modern times, Alfred North Whitehead said: "the safest general characterization of the European philosophical tradition is that it consists of a series of footnotes to Plato."

Plato was born between 428 and 423 BC into an aristocratic and influential Athenian family; through his mother, Perictione, he was a descendant of Solon, a statesman credited with laying the foundations of Athenian democracy. Diogenes Laertius asserts that that Plato is a nickname that alludes to his robust figure, and that his birth name was Aristocles (Ἀριστοκλῆς), meaning 'best reputation', but this is widely regarded as false by modern scholarship. Plato had two brothers, Glaucon and Adeimantus, both of whom appear in the Republic, and also a sister, Potone, and a half brother, Antiphon.

During Plato's childhood, Athens was involved in the Peloponessian War against Sparta. His older brothers, Adeimantus and Glaucon, distinguished themselves at the battle of Megara in 409 BC. Despite the war, Plato and his brothers, like all male citizens of Athens, received a traditional education in gymnastics and music. According to the ancient writers, there was a tradition that Plato's favorite employment in his youthful years was poetry: he wrote poems, dithyrambs at first, and afterwards lyric poems and tragedies (a tetralogy), but abandoned his early passion and burnt his poems when he met Socrates and turned to philosophy. There are also some epigrams attributed to Plato, but these are now thought by some scholars to be spurious.

In his youth, Plato first encountered Socrates, who would become his teacher and greatest source of inspiration, initially in the company of other Athenian boys in the Palaestra, such as is depicted with Lysis and Menexenus, who discuss philosophy with Socrates in the Lysis, but he soon would become a member of Socrates' inner circle, meeting with Socrates and his other followers. Socrates, along with the sophists of his day, challenged the prevailing focus of Early Greek philosophy on Natural philosophy, and investigated questions of ethics and politics, examining the ideas of his interlocutors with a series of questioning called the Socratic method.

Socrates' immense influence on Plato is clearly borne out in Plato's dialogues: Plato never speaks in his own voice in his dialogues; every dialogue except the Laws features Socrates, although many dialogues, including the Timaeus and Statesman, feature him speaking only rarely. Leo Strauss notes that Socrates' reputation for irony casts doubt on whether Plato's Socrates is expressing sincere beliefs. Xenophon's Memorabilia and Aristophanes's The Clouds seem to present a somewhat different portrait of Socrates from the one Plato paints. Aristotle attributes a different doctrine with respect to Forms to Plato and Socrates. Aristotle suggests that Socrates' idea of forms can be discovered through investigation of the natural world, unlike Plato's Forms that exist beyond and outside the ordinary range of human understanding. The Socratic problem concerns how to reconcile these various accounts. The precise relationship between Plato and Socrates remains an area of contention among scholars.^{[page needed]}

According to the Seventh Letter, whose authenticity has been disputed, as Plato came of age, he imagined for himself a life in public affairs. In 404, Sparta defeated Athens at the conclusion of the Peloponessian war, leading to the election of the Thirty Tyrants, which included two of Plato's relatives, Critias and Charmides. Plato himself was invited to join the administration, but declined, and quickly became disillusioned by the atrocities committed by the Thirty, especially when they tried to implicate Socrates in their seizure of the democratic general Leon of Salamis for summary execution.