Plethon revived Platonism against the Aristotelian orthodoxy of Byzantine philosophical education in his treatise "On the Differences Between Plato and Aristotle," and in his "Book of Laws" proposed a reformed, rationalized Hellenic religion to replace Christianity, a project so radical that most copies of the work were burned as heretical after his death. His lectures at the Council of Florence introduced Italian humanists directly to Plato's philosophy.
Intellectual setting
Plethon taught in Mystras during the final century of Byzantine rule as the empire's territory contracted to fragments, and his 1438-39 visit to Florence for the council convened to discuss reunion of the Eastern and Western churches placed him directly among the Italian humanists eager for direct access to Greek philosophical texts.
Key ideas
Platonism against Byzantine Aristotelianism
Plethon argued that Plato's philosophy was both truer and more compatible with a rationally ordered cosmos than the Aristotelianism dominant in Byzantine education, provoking a sharp rebuttal from the Aristotelian George (Gennadios) Scholarios.
A reformed Hellenic religion
In the "Book of Laws," Plethon sketched a rationalized polytheistic religion grounded in Platonic metaphysics as a proposed replacement for Christianity, a project so heterodox that the surviving Byzantine Church had almost the entire work destroyed after his death.
Legacy
Plethon's Florence lectures on Plato are credited with directly inspiring Cosimo de' Medici to found the Platonic Academy in Florence under Marsilio Ficino, making Plethon a key catalyst for the Renaissance revival of Platonism in Western Europe.