Socrates

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

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

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

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