Pyrrho

Pyrrho was, according to the First Doctor, the founder of scepticism. While visiting the planet Marinus, the Doctor advised Tarron, a policeman in Millennius, to read Pyrrho's works, as they would be helpful to someone in his line of work. When asked if he had read Pyrrho, the Doctor claimed to have met the philosopher in person. (TV: "The Keys of Marinus")