Platonism is an ancient Greek school of philosophy from the Socratic period, founded around 387 B.C. by Socrates' student and disciple, Plato, and continued by his students and followers. It was based in the Academy, a precinct containing a sacred grove outside the walls of Athens, where Plato delivered his lectures (the protoype for later universities). Platonism was originally expressed in the dialogues of Plato, in which the figure of his teacher, Socrates, is used to expound various doctrines.
Plato's philosophy is best known for its Platonic Realism (also, confusingly, known as Platonic Idealism), its hylomorphism (the idea that substances are forms inhering in matter) and its Theory of Forms ("Forms" are the eternal, unchangeable, perfect universals, of which the particular objects we sense around us are imperfect copies). It poses an eternal universe, and describes idea as prior to matter, so that the substantive reality around us is only a reflection of a higher truth. (see the section on Platonic Realism for more details).
Platonic Epistemology holds that knowledge is innate, and the immortal soul "remembers" its prior familiary with the Forms ("anamnesis"). Learning is therefore the development of ideas buried deep in the soul. Of these, the Form of "the Good" (the ideal or perfect nature of goodness) is the ultimate basis for the rest, and the first cause of being and knowledge. Plato held that the impressions of the senses can never give us the knowledge of true being (i.e. of the Forms), which can only be obtained by the exercise of reason through the process of dialectic (the exchange of arguments and counter-arguments, propositions and counter-propositions).
Platonic Ethics is based on the concept that virtue is a sort of knowledge (the knowledge of good and evil) that is required to reach the ultimate good ("eudaimonia" or happiness), which is what all human desires and actions aim to achieve (see the section on Eudaimonism). It holds that there are three parts to the soul, Reason, Spirit and Appetite, which must be ruled by the three virtues, Wisdom, Courage and Moderation. These are, in turn, all ruled by a fourth, Justice, by which each part of the soul is confined to the performance of its proper function.
The Academy, in which the school was based, is usually split into three periods: the Old, Middle, and New Academy. The chief figures in the Old Academy were: Plato's most famous student, Aristotle, who rapidly developed his own set of philosophies and a whole separate Aristotelian tradition; Speusippus (407 - 339 B.C.), Plato's nephew, who succeeded as head of the school after Plato's death in 347 B.C.; Xenocrates (396 - 314 B.C.) who was head from 339 B.C. to 314 B.C.; Polemo, from 314 B.C. to 269 B.C.; and Crates, from 269 B.C. to 266 B.C. After this time, the Middle Academy and New Academy were more vehicles for Skepticism than Platonism proper, before being re-founded, after a lapse during the early Roman occupation, as a Neo-Platonist institution in 410 A.D.
Around 90 B.C., a period known as Middle Platonism began, when Antiochus of Ascalon (c. 130 - 68 B.C.) rejected Skepticism, and propounded a fusion of Platonism with some Aristotelian and Stoic dogmas. Philo of Alexandria can also be considered a Middle Platonist, as he attempted to synthesize Platonism with monotheistic religion, anticipating the Neo-Platonism of later philosophers such as Plotinus.
Platonism influenced Christianity first through Clement of Alexandria (c.150 - 216 A.D.) and Origen (c. 185 - 254 A.D.), and especially later through St. Augustine of Hippo, who was one of the most important figures in the development of Western Christianity. Platonism was considered authoritative in the Middle Ages, and many Platonic notions are now permanent elements of Latin Christianity, as well as both Eastern and Western mysticism.