He steps from catwalk to metal grating, setting the rubber mallet gently on the central console. Switches. Levers. Buttons. The rotor wheezes and the TARDIS lifts off. Its pilot shivers.
Contrary to popular belief, he had paid some attention to his philosophy tutors. Issues of identity had (he now regrets) hardly piqued either the tutors' or the student's interest; both had preferred to focus on ethics. Now he remembers a puzzle, posed in passing and never answered to anyone's satisfaction:
There is a lost and lonely star, swept unimaginably far from the distantly glimmering galaxies. It reaches the end of its life, expands with a last monumental breath, shivers… collapses. Explodes. Its death-convulsion ejects all its mass into a swirling cloud. The cloud swirls (improbably) and re-settles, re-condenses, re-ignites. No substance has been lost, nor any gained.
Is the new star the same as the old?
The console whirrs and the rotor Dopplers down. He puts his thoughts aside a moment to gather another reward. Another glimpse of his magnificent Sarah Jane is the aching sweetness of a single drop of nectar to the tongue… never enough to satisfy. Did she understand?
He coaxes his ship into another flight. The long hands move with unaccustomed gravitas. That manic energy is diverted inward now, quelling the mad grief and bewildered rage long enough to gather a few more honeysuckle-blossoms.
He always was a star, burning hot and exuberant as no other Time Lord ever did. Now he meets the fate of all the brightest stars: supernova. He has lost and outlasted his race's galaxies. His re-ignition is certain, written in his every atom.
(He shudders, unable for a moment to hold fast against the cold fire clutching at his hearts. His last breaths burn.)
The new Doctor will not be the same as the old.
He knows exactly where the other man has been, and the lightest brush of minds was enough to confirm it. A few words on a note were enough to act on it. Jack will be happy, at least for tonight; Alonso was alone, too, in the aftermath of the Titanic. The two unsettled souls will comfort each other, brash confidence and innocent honesty balm to each other's pain.
The TARDIS doors close behind him; their click could be Atropos' scissors closing, sundering the paths of two mortal strands.
He learned another puzzle in his time among the humans–very like his tutors' star, it was:
The Ship of Theseus looms in its museum cradle. Night after night, a clever thief sneaks in and steals a single plank from its hull, replacing it with a perfect replica. Bit by bit (a fitting, a sail, a length of rope), the original ship is replaced. Eventually there are two ships–one in the museum, in the mold of the original but with none of the same parts, and one in the thief's warehouse, built from scratch but of the original parts.
Which is the true ship? Does it even still exist?
Joan's daughter's eyes undo him. Dismantle him. He holds together until the TARDIS doors close before falling to his knees, bowing his head over the console, and going to pieces. The force of his silent grief is worse, for a time, than the force of the spasms that sear along his nerves.
When he comes back to himself he feels emptied. His impending death, stealthy and larcenous, has snatched another piece of his hull and replaced it. Something new is stealing over him: treacherous shadows of a future self.
The new self is not this true Doctor. He will no longer exist.
He delivers his Donna's wedding present: lifelong prosperity, disguised as mere luck. Only he could give her such a gift, and only Wilf will ever know. Tangibly bitter irony floods his mouth.
He has tried to lie to himself, thinking that it is not so bad to lose Donna this way. He did already lose her once, after all. Isn't this supposed to lose its sting, with repetition? Does fading memory dull the pain? (Of course not. He will never be inured to loss.)
Thoughts of memory raise another puzzle:
The aging general looks back over his life. He remembers vividly how he was as a young lieutenant, and he knows that as a lieutenant he remembered vividly how he was as a schoolboy. Now, though, the general hardly remembers his boyhood at all: his childhood is a scattering of sense-impressions and a vague continuity. Winter sky and a chalk-stained hand, his brother's face and fireflies. What link remains between his old brittle bones and the lissom, whooping lad? Is he the same person now that he once was?
Swallowing around the splinters lodged in his throat, the Doctor strokes his ship's rotor. Her song caresses him back. One stop left to make. He will see Rose for the first and final time… and then he will forget. Already he is the young lieutenant, aging far too fast: his boyhood slips from memory even as the strength leaches from his arms.
The Doctor sways as he turns towards the door. Weakness lifts the top off his brain, sends him spiraling into the white embrace of crushing pain. When he comes back to himself he is slumped against the rail, his hearts stammering. Not long now. The star is collapsing, the ship crumbling, the memories fading. There's no time.
On the other side of that door waits the last act of closure before his change. Regeneration. He's always told his companions that Time Lords can cheat death, that regeneration is just a superficial change. He knows better. It won't be the same star, the same ship, the same man. He is dying.
He doesn't want to go.