In The Doctor Falls, the Doctor

reprograms the cybermen's network to hunt for beings with two hearts

And it seems to work well. How come afterwards the Cybermen are still searching for humans to upgrade?

The simplest answer is that one of the not-yet-converted humans (like the surgeon) found the issue and fixed it. Is this addressed in the episode?

  • I'm not sure he reprograms cybermen to chase only beings with 2 hearts. (this is briefly discussed here: scifi.stackexchange.com/questions/163532/… in the comments)
    – elrond
    Commented Jul 25, 2017 at 21:54
  • 3
    I believe he said something about expanding the definition of human. He reprogrammed them to look for hearts: Min: 1, Max: 2. A better question would be why was the number of hearts even a criterion for identifying humans?
    – J Doe
    Commented Jul 25, 2017 at 23:42

1 Answer 1


From "The Doctor Falls" Script: (emphasis mine)

DOCTOR: You shouldn't have hit me, Missy. I was waiting for my chance. Computer, containing the algorithm defining human life signs. I only had time to change one detail. A single number. One to a two. One heart to two hearts. I expanded the definition of humanity. Took 'em a while to update the net, but here we go. Welcome to the menu.

The Doctor didn't change the definition of a human to having two hearts, he EXPANDED it. Basically, he told the Cybermen that a human could have one or two hearts, so they proceeded to hunt both humans and timelords.

  • Great answer. However, this raises the question, why did he not change it so that the cybermen would leave the innocent people alone and only chase after missy, the master and the doctor?? It seeems like a doctory move
    – HN17
    Commented Dec 23, 2017 at 20:34
  • 1
    @HN17 Because he "only had time to change one detail", as it says in tilley's quote.
    – Rand al'Thor
    Commented Oct 5, 2018 at 8:42
  • @Randal'Thor Which raises the question of how the software works. You'd think it would specify an exact number of hearts, but the quoted explanation only works if it specifies a maximum number of hearts. What's more, the GUI must have told the Doctor that fact.
    – J.G.
    Commented Feb 27, 2020 at 21:20

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