Dunno If My Math Is Right But That Makes Theta Sigma Number 186 If They Go Down The Greek Alphabet Like That - Tumblr Posts

Random thought: what if Theta Sigma isn't just an Academy nickname, but a designation given by the Time Lords to denote which iteration of the Doctor they're dealing with?

The Doctor doesn't know that, they think it's just a silly Academy thing. They don't know that each time the Time Lords have wiped their memory they've been given a marker: Alpha Alpha, Alpha Beta, Alpha Gamma... all the way until Theta Sigma.