Conceived of by the BBC's then head of drama, Sydney Newman, and Donald Wilson, head of script development, and BBC staff writer "Bunny" Webber in 1962, and delivered by producer Verity Lambert and director Mervyn Pinfield in 1963, he was, and is, and always will be known, in all his regenerations, as the Doctor.

The Doctor is not, in and of himself, a family, but he is very like one. He is, over time, the same but different. He doesn't reproduce, but does regenerate. He may have a different character, wear different clothes and talk differently, yet he is always, somehow, the same. Doctor Who may be television's longest-running science fiction series, with an ever-increasing range of spin-offs and merchandising, but it's also a tale of dynastic succession which raises the most fundamental questions about personal identity. Who am I? Where am I from? And which way back to Gallifrey?

In their just published but already indispensable Doctor Who and Philosophy (2011), Courtland Lewis and Paula Smithka gather together a number of philosophers to tease out some of the important questions posed by the programme, such as Should the Daleks Be Exterminated?, Is the Doctor the Destroyer of Worlds? and most important, Who Is the Doctor? For That Matter, Who are You?

In his essay in the book, Just As I Was Getting To Know Me, Patrick Stokes defines the continuing appeal of the Doctor as "the idea of a greatly extended lifespan with bodily regeneration". The story of the Doctor's regeneration was, in fact, invented by the editors as a way of replacing the first Doctor (1963-1966), William Hartnell, while allowing the series to continue. This neat sleight-of-hand allowed Patrick Troughton to become the second doctor (1966-1969), Jon Pertwee the Third (1969-1974) and Tom Baker as the Fourth and, arguably, the best.

Baker played the part for a record seven years, in a fedora and an unfeasibly long scarf. His doctor was somehow deeply serious, and yet at the same time clearly crackers. In an interview in Doctor Who magazine, Baker admitted: "When I did Doctor Who, there was no question of acting. I'm not very good at acting. What I'm quite good at, I think, is performing." After Baker there came, as in all dynasties, the inevitable decline. Peter Davison played the fifth doctor (1981-1984) wearing celery in his lapel. Colin Baker played the sixth (1984-86) in a horrid patchwork suit. Sylvester McCoy was the seventh (1987-89), in a panama and a pullover, and Paul McGann was the eighth, in a made-for-TV movie (1996).

The doctor's fortunes revived when Christopher Ecclestone returned as the ninth incarnation, in a leather jacket, in 2005, swiftly succeeded by a besneakered David Tennant as the 10th (2005-2010), and now Matt Smith in a bow-tie as the 11th. By Time Lord lore, the Doctor can only regenerate up to 12 times, meaning there can be only 13 doctors in total, leaving us with two more to go.

There have been occasional family get-togethers. In a 10-year anniversary special in 1973, Hartnell, Troughton and Pertwee starred in an episode called The Three Doctors. In 1983, Troughton, Pertwee, Davison and Baker starred in The Five Doctors, with another actor playing the deceased Hartnell's role. And there are his companions – Jamie, Tegan, Sarah Jane, Rose Tyler, K-9. And the moral of this story for students of the family? Perhaps simply this: in the future, our role will be played by others.