The following contains speculation based on information already available in the Doctor Who universe. Ahead of the Christmas special The Time of the Doctor, that speculation could potentially constitute mild #spoilers for some people
Ever since John Hurt arrived on the scene back in May in the Doctor Who series seven finale, confusion has reigned as to how the various incarnations of the Time Lord should now be numbered. Hurt’s Doctor, it has since been confirmed, was the immediate successor of Paul McGann’s eighth Doctor, who was never actually seen regenerating (Christopher Eccleston’s “ninth Doctor” appeared fully formed when the series returned in 2005).
Showrunner Steven Moffat’s initial reaction was that, since the actions of Hurt’s “War Doctor” (destroying his home planet of Gallifrey in order to put an end to the bloody Time War) were not performed “in the name of the Doctor”, numbering should stay the same: Matt Smith would remain 11 and his successor Peter Capaldi would be 12.
But as The Time of the Doctor approached, Moffat told us that the Christmas special would see the Doctor staring death in the face, having run out of regenerations. Why…?
Standard issue for a Time Lord is 12 lots of regeneration energy, which, all things being equal, means 13 different faces before they kick the (rather generous) bucket. That would mean new ninth Doctor John Hurt would bump his predecessors along one – David Tennant would become 11, Matt Smith would become 12 and Peter Capaldi’s 13th Doctor would be the last.
However, just to complicate things, Moffat then reminded us that David Tennant’s Doctor had used an extra helping of regenerative energy to grow a copy of himself from his severed hand in series four finale Journey's End (it's a long story). This means that, although Smith was only the 12th face of the Doctor, he had already used up his final regeneration and the Doctor will die with Matt Smith.
But of course, that’s not going to happen…
"The 12 regenerations limit is a central part of Doctor Who mythology – science fiction is all about rules, you can't just casually break them," Moffat told RadioTimes.com recently.
"So if the Doctor can never change again, what's Peter Capaldi doing in the Christmas special?"
What indeed? Well... it was established some time ago, in 20th anniversary The Five Doctors, that the restriction of 13 incarnations was not immutable but able to be overturned by the Time Lord council.
In fact, that story set a precedent for more lives being granted, with the Doctor’s dying foe the Master handed a whole extra cycle of 13 generations in return for carrying out the Gallifreyans' dirty work. Whether this is what will happen to Matt Smith’s Doctor we don’t yet know, but it at least hints at a way out (and hopefully means the whole Peter Capaldi thing wasn’t just a publicity stunt).
But where does that leave us in terms of what we call the Doctors? Moffat says Matt Smith is number 13, and in terms of regeneration energy that’s true. But he’s only had 12 different faces and 12 personalities and the Tennant doppleganger now resides in a parallel universe (again, a long story) so is out of the picture. Unless the BBC is going to add an extra David Tennant to the line-up in the official image above, I think that makes Matt Smith the 12th Doctor and Peter Capaldi the 13th.
But that's just my view. After The Time of the Doctor has been and gone, Who knows what we'll be saying...