<P> The number of previous incarnations of the Doctor was at first unclear within the series . In the Fourth Doctor story The Brain of Morbius, the Doctor participates in a mental' duel' with another Time Lord and the machine to which their minds are connected begins to project the faces of the "losing" contestant's regenerations in chronologically descending order . As the Doctor is overpowered by Morbius, the images change successively to those of the third, second and first Doctors, then eight further faces appear . It was the intention of producer Philip Hinchcliffe and script editor Robert Holmes that images were even earlier incarnations of the Doctor . However, the narrative does not explicitly confirm that the faces do represent incarnations of the Doctor, even though, as the faces are shown, Morbius cries "How far Doctor? How long have you lived?", indicating that they are not incarnations of Morbius, but rather that it is the Doctor's past being shown . </P> <P> In other episodes, it was firmly established that the William Hartnell incarnation of the Doctor was the very first . In The Three Doctors, the Time Lord President describes the Hartnell incarnation as "the earliest Doctor". This is cemented in The Five Doctors when Peter Davison's Fifth Doctor (introducing himself to the First Doctor) says that he is the fourth regeneration, meaning that there have been five of him . The First Doctor also refers to himself as "the original". Episodes of the revival series showing the lives of the Doctor--"The Next Doctor", "The Eleventh Hour", "The Day of the Doctor" and "The Husbands of River Song"--all begin with the William Hartnell incarnation . </P> <P> In "The Lodger", after the Eleventh Doctor shows Craig who he is, he points to his face and says, "Eleventh". However, in "The Name of the Doctor", when the Doctor rescues Clara from inside his own time stream, they both see another figure that Clara doesn't recognise . The Doctor then reveals that this is a past incarnation that he deemed unworthy of the name "Doctor" due to the atrocities he committed during the Time War, and only accepted him after learning the truth of how the war ended ("The Day of the Doctor"). Ultimately, in "The Time of the Doctor" the Eleventh Doctor reveals that, counting the War Doctor and the Tenth's aborted regeneration, he is actually in his final incarnation, reaching a point where he is dying of old age after centuries of conflict with the Daleks and others on the planet Trenzalore . However, at the behest of Clara, the Time Lords grant the Doctor a new regeneration cycle, allowing a thirteenth regeneration into an incarnation known as the Twelfth Doctor . </P> <P> In "The Time of the Doctor", the Eleventh Doctor describes his new regeneration ability as the start of a new' cycle', implying that he's been restored to the customary twelve regenerations . However, in "Kill the Moon", the Twelfth Doctor says he's "not entirely sure (he) won't keep regenerating forever," once again raising the question regarding any limits to this ability . In "Hell Bent", Rassilon asks the Doctor "How many regenerations did we give you?", and during "The Doctor Falls", two incarnations of the Master express uncertainty about how long it would take them to kill the Doctor, further implying that the Doctor has a finite number of regenerations even as the exact amount remains undetermined . </P>

How was the doctor able to regenerate 13 times
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