Plato was an ancient Greek writer and philosopher and was the student of another known philosopher, Socrates.
The Doctor's companion Vicki had never heard of either Plato or Socrates. (PROSE: Byzantium!)
The Fourth Doctor and Leela met Plato at a gathering at Anaximander's home. (PROSE: The Brain of Socrates)
The Library of St John the Beheaded contained some of his lost works. (PROSE: All-Consuming Fire)
Bernice Summerfield met Plato and his teacher Socrates whilst time-travelling. (AUDIO: The Oracle of Delphi)
The Eleventh Doctor, Amy Pond and Rory Williams also met him and his mentor. (COMIC: The Chains of Olympus)