Socrates

Socrates was an ancient Greek writer and philosopher and was the mentor of another known philosopher, Plato.

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

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

The Doctor, in his sixth incarnation, mentioned that he met the philosopher, telling him to "stick to his guns" when it came to his theories. (AUDIO: The Marian Conspiracy)

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