It has been referred to in recent episodes as the oldest question in the universe, the question that has been a running gag throughout the fifty year history of the show: Doctor who? It is implied that he has a name, but for whatever reason he won’t tell anyone what it is. He’s just the Doctor to everyone he meets. The implications of what it means to be the Doctor are so tied to his identity that he even had one incarnation that refused the title because he couldn’t stand to have it associated with the one who ended both the Time War and the whole Time Lord race. In “The Day of the Doctor,” the name is described as a promise: “Never cruel or cowardly. Never give up, never give in.” It lets humans know that help has arrived and it strikes fear into the hearts of Daleks, Cybermen, and a myriad of other monsters and bad guys.
There is perhaps an older question in the universe, the one that Moses asked: God who? In the ancient world so full of gods, who are you that people should listen to the message I bring from you? It’s funny how Moses receives the same kind of answer as so many companions over the years. Just as the Doctor is simply that, God just IS. It’s a name that holds a promise: eternal, ever-present, and all-powerful. I AM - that’s all the answer you’re going to get, but it’s all the answer you need.