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Categorical Imperative was the seventh short story in the Short Trips anthology Short Trips: Monsters. It was written by Simon Guerrier. It featured the First Doctor, Second Doctor, Third Doctor, Fourth Doctor, Fifth Doctor, Sixth Doctor, Seventh Doctor, Eighth Doctor, Sarah Jane Smith, Ace, Charley Pollard, Jamie McCrimmon, Jo Grant, Susan, Peri and Tegan.


The Doctor has brought Sarah to a christening, but he seems in a bad mood. Sarah entertains herself by watching the queue of people waiting to give the baby his gifts. She notices that the baby's mother, who is named Ann, is surrounded by several young girls. Sarah wonders about their purpose, but when she nudges the Doctor, he says, "I know. Don't let him see you". Surprised, she looks at the people in the queue, and is shocked to see the old Doctor that she knew before. She has a sudden suspicion about the baby.

The Doctor had taken Sarah to Earth far in her future. It had suffered a cataclysmic disaster, long enough ago that things had started to decay. After much searching, they found survivors. They tried to help, but there was little they could do. As they leave, Sarah asked to go to a place where they could make a difference.

The Third Doctor carefully removes a silver knife from the package and prepares to use it. Jo is at the head of the queue with the mother, trying to distract her. An old man is at the front of the queue. He accidentally drops his present, and a silver knife falls out. He grumbles a bit and leaves, accompanied by his granddaughter, who had been up front with Ann.

Sarah realises what the Third Doctor is about to do, and is appalled. The Fourth Doctor tells her that in fifty-two years, the baby will be murdered by his aides for the devastation he caused.

At the front of the queue, Ann is yelling at an American girl. The Third Doctor is at the front of the queue. He glances back, and sees behind him in the queue a large blond man. He notices that the man also has a silver knife, and is relieved that he doesn't have to be the one to kill the baby. He and Jo leave.

While Peri takes her turn distracting Ann, Sarah wonders why the Third Doctor didn't go through with it. She sees an eccentric-looking blond man in the queue and asks her Doctor if that's him as well. As she looks down the queue, she sees several other eccentric looking men — a little guy in a straw hat, a young man in a beige coat and a man with a flashy silver cravat — and they are all carrying a silver knife. She wonders if they could all be Doctors.

Underneath the chamber, the Doctor's TARDIS sits in a corner. The Fifth Doctor and Tegan enter and it dematerialises, but when the sound dies down, the TARDIS is still there. The Sixth Doctor and Peri arrive and enter, and the same thing happens again. Events repeat themselves when the Seventh Doctor and Ace arrive.

Back in the chamber, Charley is talking to Ann. The Eighth Doctor is moving up the queue. The Fourth Doctor finally realises how to solve the dilemma. He sends Sarah off to Ann, and heads off.

The Eighth Doctor approaches the baby, and Ann realises that he means to kill him. She yells for the guards, who subdue the Doctor. Ann looks at the face of her baby, and realises, for the first time, how much he means to her.

Sarah is waiting below at the TARDIS. When the Fourth Doctor arrives, he explains that he helped the other Doctor get away. Sarah tells him that the other Doctor explained to her that things would be better, that the mother's affection would change the way the baby grew up. She also tells him that the other Doctor's TARDIS was parked on top of theirs, and that he and the girl Charley have already left. Sarah and the Doctor enter the TARDIS and it dematerialises, but this time, there are no police boxes left behind.



  • The Eighth Doctor is wearing a silver cravat.


  • This story is named after Immanuel Kant's categorical imperative, the central concept in his moral philosophy, in which choices and actions should be dictated by purely logical imperatives, or "pure reason". One of the Doctors killing the baby in this story would be following utilitarianism, or indeed consequentialism, whereas Kant's categorical imperative, an example of deontology, would hold that killing is wrong in all circumstances. A Kantian argument would maintain, basically, that the ends do not justify the means in this scenario, as the requirement not to kill must be obeyed in all circumstances.
  • Sarah argues on Kant's side, whispering to the Doctor, "You can't kill a child!" She makes clear that she thinks this should hold in all circumstances.
Doctor: "You know what he'll grow up to do."
Sarah: "But that doesn't make it right!"
  • The Doctor says that Sarah would get along with Kant.
  • The Fourth Doctor faced a similar dilemma, of deontology vs. consequentialism, in Genesis of the Daleks, where he asks himself, with the power to destroy all Daleks and so prevent all their forthcoming destruction, "Do I have the right?" As here, the Doctor has foreknowledge of the many deaths he can prevent, but decides, in the end, not to take action.