Georgius Gemistos Plethon (or Pletho), (c. 1355-1452), Greek Neoplatonist philosopher and scholar, one of the chief pioneers of the revival of learning in Western Europe, was a Byzantine by birth who settled at Mistra in the Peloponnesus, the site of ancient Sparta.

As a young man he began to study Plato, and was so enamoured with the philosopher that he took the similar-sounding name Plethon ("the full"). He was likely influenced in his study by the Muslim scholars in the Ottoman Empire, which had its capital at Adrianople. The Muslims had very early on inherited Byzantine scholarship on ancient philosophy, and some of this knowledge was probably retransmitted to Byzantine scholars, who had their own traditional interpretations.

Plethon was the author of De Differentiis, a description of the differences between Plato's and Aristotle's conceptions of God. Scholarios later defended Aristotle and convinced emperor Manuel II Palaeologus that Plethon's support for Plato amounted to heresy. Manuel had Plethon confined in Mistra, though he remained somewhat of a celebrity. In Mistra he wrote pamphlets to Manuel II describing how the Empire could be reorganized according to Plato's Republic. He also wrote a Summary of the Doctrines of Zoroaster and Plato, which detailed his own pseudo-polytheistic beliefs. Knowledge of Zoroastrianism most likely came from contact with Muslim scholars. These works did not help to clear him of the charge of heresy. He also wrote about the condition of the Peloponnesus, compiled several volumes of excerpts from ancient authors, and wrote a number of works on geography, music and other subjects.

Byzantine scholars had been in contact with their counterparts in Western Europe since the time of the Latin Empire, and especially since the Byzantine Empire had begun to ask for Western help against the Ottomans in the 14th century. The West had some access to ancient Greek philosophy through the Roman Catholic Church and the Muslims, but the Byzantines had many documents and interpretations that they had never seen before. Byzantine scholarship became more fully available to the West after 1438, when Byzantine emperor John VIII Palaeologus attended the Council of Ferrara and the Council of Florence to discuss a union of the Greek and Roman churches. Accompanying John VIII were Plethon, his student Johannes Bessarion, as well as George Scholarios (the future Patriarch Gennadius II).

As a secular scholar Plethon was often not needed at the council. Instead, he set up a temporary school to teach interested Florentines about previously unknown (to them) works of Plato. He essentially introduced Plato to the Western world, and shook the exclusive domination which Aristotle had exercised over European thought for eight centuries. Cosimo de' Medici attended these lectures and later founded an academy in Florence, where Italian students of Plethon continued to teach after the conclusion of the council. Because of this Plethon is considered one of the most important influences on the Italian Renaissance.

He died in Mistra in 1452, just before the fall of Constantinople to the Ottomans. His Summary, considered the most heretical of his works, was later burned by Gennadius II. Many of his other works still exist in manuscript form in various European libraries. Most of Pletho's works can be found in JP Migne, Patrologia Graeca, collection; for a complete list see Fabricius, Bibliotheca Graeca (ed. Harles), xii.

This article incorporates text from the public domain 1911 Encyclopædia Britannica.