CLARA: Have we just watched the entire life cycle of Earth, birth to death?
DOCTOR: Yes.
CLARA: And you’re okay with that?
DOCTOR: Yes.
CLARA: How can you be?
DOCTOR: The Tardis, she’s time. We. Wibbly vortex and so on.
CLARA: That’s not what I mean.
DOCTOR: Okay, some help. Context? Cheat sheet? Something?
CLARA: I mean, one minute you’re in 1974 looking for ghosts, but all you have to do is open your eyes and talk to whoever’s standing there. To you, I haven’t been born yet, and to you I’ve been dead one hundred billion years. Is my body out there somewhere, in the ground?
DOCTOR: Yes, I suppose it is.
CLARA: But here we are, talking. So I am a ghost. To you, I’m a ghost. We’re all ghosts to you. We must be nothing.
DOCTOR: No. No. You’re not that.
CLARA: Then what are we? What can we possibly be?
DOCTOR: You are the only mystery worth solving.