Plato is celebrated as one of the most significant early Greek philosophers. His theories of form, ethics and politics, in particular, have proven enormously influential in both Western and Islamic traditions. Some, such as Alfred North Whitehead, have even considered all subsequent Western thought as simply a "footnote" to Plato.
Plato's most significant works come in the form of dialogues, often with the main character being Plato's own teacher, Socrates. The dialogues are typically structured with Socrates engaging in discussions with one or more interlocutors, wherein Socrates exposes their lack of command over concepts they claimed to have actually mastered. In many cases, Plato's Socrates is trying to come to grips with the nature of virtue, attempting to supply accurate definitions for qualities such as piety, courage and goodness. Some of Plato's most famous dialogues are his political treatise, "The Republic," his accounts of Socrates' trial and death, "The Apology" and "The Phaedo," and his exposition of the nature of love in "The Symposium."
With his metaphysics, Plato suggests a host of primordial and transcendent forms in which the physical beings of concrete existence participate. For example, for all chairs encountered in the physical world, there is an original cosmic design from which that real chair is derived. Plato expands on this to introduce pure primordial forms of the virtues aforementioned, ones that human beings then try to emulate or embody.
In addition to his writing, Plato himself was a great teacher of future minds. His most famous student, Aristotle, eventually became the other great towering mind of classical Greek philosophy. Despite exhibiting consistent differences with Plato's, Aristotle's thought likely would have never achieved its own particular shape without the guidance of his famous mentor.