Platonism

Plato's central and most debated idea is that particular things in the world, a beautiful face, a just decision, an equal pair of sticks, are beautiful, just or equal only because they participate in an unchanging, non physical Form of Beauty, Justice or Equality that exists independently of them and of any mind that thinks about it. The senses report only the shifting particulars, so for Plato genuine knowledge, as opposed to mere opinion, has to be knowledge of the Forms themselves, reached through reasoned argument rather than observation. The most famous illustration is the allegory of the cave in the Republic, where prisoners who have only ever seen shadows on a wall mistake the shadows for reality, and philosophy is described as the difficult climb out of the cave toward the sunlight of the Forms. The theory answered a real problem, how a word like equal can apply correctly to many different pairs of things that are never perfectly equal to each other, but it also raised the so called third man problem, an infinite regress worry that Plato himself records in a late dialogue, the Parmenides, without fully resolving it. Aristotle, who studied under Plato for two decades, eventually rejected the theory outright, arguing that the essence of a thing is not stored in a separate realm but is present within the thing itself, a disagreement that set the terms for most of the metaphysics that followed both of them.

Plato's Academy did not remain a fixed doctrine after its founder's death in 347 BCE. Leadership passed first to his nephew Speusippus, then to Xenocrates, both of whom continued broadly dogmatic Platonic teaching, but in the third century BCE the school turned sharply toward skepticism under Arcesilaus, who argued that Socrates' own admission of ignorance was the true spirit of the Academy and that certainty should be withheld on every question. This skeptical phase, later called the New Academy, reached its most systematic form under Carneades in the second century BCE, who developed sophisticated arguments against Stoic claims to certain knowledge while famously visiting Rome and arguing both sides of the question of justice on consecutive days, to the alarm of Roman moralists. The school returned to a more positive, dogmatic Platonism under Antiochus of Ascalon in the first century BCE, who blended Platonic, Aristotelian and Stoic ideas into what he presented as a recovery of the Academy's original teaching. The physical school itself was destroyed during the Roman general Sulla's siege of Athens in 86 BCE, but the philosophical tradition continued under the name Platonism for centuries afterward, eventually feeding into the very different, more mystical Neoplatonism associated with Plotinus in the third century CE.