Plato

Plato was a Ancient Greek writer and philosopher and was the student of another known philosopher, Socrates.

The Doctor's companion, Vicki previously never heard of either Plato or Socrates. (PDA: Byzantium!)

The Library of St John the Beheaded contained some of his lost works. (NA: All-Consuming Fire)

Bernice Summerfield met Plato and his Socrates whilst time-travelling. (BFBS: The Oracle of Delphi)

The Eleventh Doctor, and his companions Amy Pond and Rory Williams, also met him and his mentor. (The Chains of Olympus)