One of television’s greatest mysteries is the name of one world-famous Time Lord. For years, viewers have speculated about the Doctor’s true name in BBC’s long-running series, Doctor Who.

The Doctor has had many aliases and nicknames throughout the years but fans continue to wonder what his true name might be. Doctor Who has been airing since 1963, and while the show has obviously gone through many iterations and Doctors, one thing is consistent: we still don’t know the Doctor’s true name.

Fans have developed theories, and some believe the name could be hiding in plain sight while others think it has to be more complicated than that. While the simplest answer is that no one knows the Doctor’s actual name, we think it’s more fun to ponder the possibilities.

What’s the Doctor’s real name? Popular theories explained

The Doctor is his Gallifreyan title, which prompts a cheeky “Doctor Who?” response (hence the show’s title) but could his name actually be Doctor Who? It seems a little too obvious—not to mention a bit anticlimactic—when Missy impersonates the doctor in a 2017 episode and insists it’s the Doctor’s real name that he chose in order to seem mysterious. This is obviously played for laughs, but it’s a possibility, especially after the Twelfth Doctor himself, Peter Capaldi, said it’s a valid belief.

In the same interview, Capaldi said he knows the Doctor’s name although humans can’t pronounce it, saying it’s at “a frequency that can only be heard of people with good heart.” This might be something he made up, but it tracks: the Sixth Doctor told his companion, Peri, she wouldn’t be able to pronounce his name.

The Doctor also goes by the name John Smith multiple times throughout the series, as early as the Second Doctor. Notably, the Twelfth Doctor used it when he went undercover as a school caretaker. While this could be the Doctor’s true name, it seems more likely that this is just a pseudonym he really likes using.

Showrunner Steven Moffat jokingly told readers of Doctor Who Magazine that the Doctor’s real name is “Mildred” and said each showrunner is informed of the name and “commanded never to reveal what we have learned because then the show would have to be renamed Mildred.”

Another popular theory is that the Doctor’s true name is ΘΣ, pronounced Theta Sigma. In an early episode, the Fourth Doctor runs into an old Time Academy classmate, Drax, who addressed him by the name. Many believe this is yet another of the Doctor’s nicknames, but it’s possible that’s a variation of the Doctor’s true Gallifreyan name. An official reference guide, The Making of Doctor Who, written by script editor Terrance Dicks and screenwriter Malcom Hulke, lends some credibility to this theory; the guide casually lists the Doctor’s name as δ³Σx². The guide gives other Time Lords similarly mathematical names as well, leading us to believe this could be at the very least a variation of the Doctor’s name in Gallifreyan.

Why is the Doctor’s name a secret?

Why bother keeping the Doctor’s real name a secret? It gives fans something fun to theorize, but there are other in-show reasons for not telling people his true name. If the “Timeless Child” plot is to be believed, the Doctor might not even know his original name in the first place, and whatever name the Doctor has used through many reincarnations is hidden deliberately.

The Eleventh Doctor told his companion, Clara, that his name would act as a signal to the other Time Lords, telling them it’s safe to return to this universe. Clara tries to exploit this after learning the Doctor’s real name, calling the other Time Lords to come help him as he surrendered to the Daleks. It’s implied the Time Lords do help the Doctor as he regenerates, destroying the Daleks in the process. However, the information ultimately gets wiped from Clara’s memory. This power could be detrimental in the wrong hands, so it makes sense the Doctor wouldn’t want to reveal his true name.

It’s also shown the Doctor’s name opens his tomb, which allows the Great Intelligence to access and alter the Doctor’s timeline, thus destroying all progress the Doctor had made to help the universe. That’s obviously not great, and we can understand why the Doctor wouldn’t want that knowledge to get in the wrong hands.

Whatever the truth, the Eleventh makes it clear that the name isn’t what matters. When Clara takes it upon herself to right the universe, the Doctor tells her, “My name, my real name, that’s not the point. The name I chose is the Doctor. The name you choose, it’s like a promise you make.” The Doctor chose this moniker as a way to convey that he’s here to help and to make the universe better than he found it. We may never know the answer to the age-old question, “Doctor who?” but we can agree that the “Doctor” title is more significant than any name.

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