Theories & Headcanons

Chapter 1: The Doctor's Name

LEAD-UP/EVIDENCE

Here's the two examples I could find of the Doctor's name:

d³∑x₂ or possibly Delta-Cubed Sigma X-Squared, AKA Change-Cubed Summation Unknown-Squared. From the sources I can gather, at least, so take this with a grain of salt because I am terrible at maths. 

ΘΣ or Theta Sigma, AKA Th and S. Both capitalized.

Since the first was in an in-universe public document, I'm going to assume it's a code or some such, rather than his proper name. However, I find it very interesting that both examples include a capital Sigma.

The Master's name was supposedly Koschei, which is a Russian mythical figure from what I can gather. Perhaps that means that the Doctor's name is human in origin as well.

His name is pronounceable by humans, because River once whispered it in his ear, no matter what he's claimed in the past (The Doctor lies).

Name magic exists in-universe, as seen in at least one episode.

ACTUAL THEORY

The Doctor, for whatever reason, can be controlled by his name. Which is why only River, the woman he loves and trusts most, is allowed to know it — because she wouldn't abuse it, at least not by the time she actually learns it. What's more dangerous than someone in command of the Oncoming Storm? It could as well explain why some villains are obsessed with knowing it.

ΘΣ are his initials. His name, like the letters, is Greek in origin and quite possibly shared with one or more mythical figures (wouldn't it be funny if his first name was Thanatos?).

And that's it, that's my theory. What do you think?