Arthur Conan Doyle

Sir Arthur Conan Doyle was a writer most well-known for the books about the detective Sherlock Holmes. Doyle fictionalized the adventures of Sherlock Holmes and Dr. John Watson. (NA: All-Consuming Fire)

In 1880, Doyle aided the Doctor and Sarah Jane Smith, though the Doctor suspected that they may have met before. (MA: Evolution) Indeed, in an earlier adventure the Doctor refered to him as an 'old friend'. (PDA: The Murder Game) In 1902, he met and aided the Tenth Doctor again. (NSA: Revenge of the Judoon) Some years later, he examined the famous Cottingley fairy photos and pronounced them genuine. (TW: Small Worlds)

Minor references
As a doctor, he accompanied Redvers Fenn-Cooper on an expedition to observe giant reptiles which resembled dinosaurs, though at first he had trouble believing they existed. The implication is that this would be an inspiration for The Lost World. (DW: Ghost Light)

Behind the scenes

 * Though Jack Harkness implies in Small Worlds that such a hard-minded skeptic would have trouble accepting the Cottingley fairy photos, in reality, in his later years he accepted the existence of the supernatural, including the literal existence of fairies.