VALARIE: I know what it's like to lose someone! The grief? Oh, that comes easy. The hard part is realising you can't change the past!

HAYDEN: It is a necessary evil.

DOCTOR: Oh I've met evils you wouldn't believe, gods and devils, warewolves and vampires, every monster every child's ever imagined creeping out from under the bed and you know what? They're all real. But a necessary evil - that's the only one that's a fiction.