3. The Moment
It's not only the Doctor who evolves throughout this episode but also "The Moment". At first we know it as a thing - a weapon of mass destruction. By the end of the story however, "The Moment" could stand for Gallifrey now suspended in a moment of time. Words have power and their meaning is based entirely on perception. The Doctor had to undertake this journey in order to learn that he didn't have to take "The Moment" at face value. Once he allowed himself to look at the problem from a different angle he had the power to change the meaning of those words. Death is one of the few things the Doctor doesn't really understand. It bewilders him. He doesn't share the sense of mortality that overshadows our own short lives. In a rather hilarious retcon, David Tennant's last words in this story are "I don't want to go." I'm pleased he had a chance to speak those lines in a more fitting context than the rather deplorable "End of Time". That is a central tenet of the Doctor's character, however he really doesn't want to go. If there's a way out he'll take it. His regeneration limit is fast approaching I can't wait to see how Steven Moffat deals with that. Christmas should be very interesting indeed.