Socrates

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

Biography
Socrates was used by Grimmon to repair his spacecraft. The Fourth Doctor and Leela rescued Socrates. (PROSE: The Brain of Socrates)

Bernice Summerfield and Jason Kane met Socrates and his student Plato after travelling back in time to 430 BC. (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. However, he pointed out that Socrates was unaware of what guns were. (AUDIO: The Marian Conspiracy)

The Eleventh Doctor met him again, with his companions Amy Pond and Rory Williams, in 410 BC. (COMIC: The Chains of Olympus)