Does this Doctor Who fan theory prove Ashildr isn’t really alive?

But then again, maybe the immortals’ club isn’t quite as crowded as we thought, because one new fan theory suggests that Ashildr isn’t the same kind of immortal as Captain Jack at all and might not be technically “alive” at all.

The reasoning? Well, as pointed out by redditor NextStopGallifrey, a major feature of Williams' character is the fact that no matter how long she lives she can’t remember more than a normal human lifespan. As she explains in several episodes throughout the series the only way she holds onto her past is through writing in a mass of journals, without which she couldn’t even remember Clara or her own name.

Makes sense, right? Of course it does. But there’s one niggle - why does fellow immortal Captain Jack Harkness remember everything?

Created immortal by a Time Vortex-empowered Rose in 2005 finale The Parting of the Ways, Jack has since lived thousands of years (including a couple of thousand buried alive where he couldn’t journal), with no loss of memory whatsoever. When he saw the Daleks, the Doctor and Rose in series four finale The Stolen Earth/Journey’s End he hadn’t seen then for countless centuries, but he had no trouble remembering who they were. Though from the future, Jack was originally a normal human like Ashildr – so why’s he so much better at remembering?

NextStopGallifrey has a solution – unlike Jack’s Ashildr’s longevity isn’t “true” immortality at all, and instead her revival was more like the nanogenes in 2005 two-parter The Empty Child/The Doctor Dances, described by Christopher Eccleston’s Ninth Doctor below:

“What's life? Life's easy. A quirk of matter. Nature's way of keeping meat fresh. Nothing to a nanogene.

“The nanogenes don't just fix you up, they get you ready for the front line. Equip you, programme you.”

In other words, the theory posits that the Mire chip in Ashildr’s head keeps her “meat” fresh, but doesn’t necessarily keep her mind intact. The little gas mask boy searching for his mummy in The Empty Child was brought back by warrior alien tech, just like Ashildr, and unless she obsessively journalled she could lose touch with her humanity just like he did.

It’s certainly an interesting theory – we can’t believe we never noticed the thing about Jack’s memories before to be honest – but we’ll leave it up to you to decide if it fits.

For our part, we can’t help but feel that the discrepancy in memory could come from another source, like the fact that Jack’s immortality comes from the power of the Time Vortex rather than dodgy alien tech. Perhaps that process could have shaped his brain along with his body, making it more like the minds of the similarly Vortex-exposed Time Lords?

Still, whatever the truth only one thing is certain. Captain Jack would be much better at barbecue small talk than Ashildr.