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. (PROSE: Byzantium!)

Bernice Summerfield met Socrates and his student Plato whilst time-travelling. (BFBS: 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 would meet him again, with his companions Amy Pond and Rory Williams. (COMIC: The Chains of Olympus)