Bill Potts: You dash around the universe trying to figure out what’s holding it all together, and you really, really don’t know?
First Doctor: You know me in the future, do I ever understand?
Bill Potts: No… I really don’t think you do. Everyone who’s ever met you does. You’re amazing, Doctor. Never forget that. Never, ever.
From Doctor Who – Season 11 Episode 0: ‘Twice Upon a Time’ (11×00 / Christmas special 2017)