DOCTOR: You know, you're a classic example of the inverse ratio between the size of the mouth and the size of the brain.

SARAH JANE: Doctor, you're being childish.

DOCTOR: Well of course I am. There's no point in being grown up if you can't be childish sometimes.

DOCTOR: One day, I shall come back — yes, I shall come back. Until then, there must be no regrets, no tears, no anxieties. Just go forward in all your beliefs, and prove to me that I am not mistaken in mine. Goodbye, Susan, goodbye, my dear.