Doctor Who: 20 Things You Didn't Know About The Day Of The Doctor

Just as all three incarnations of the Doctor are preparing to destroy Gallifrey, Clara reminds them of the promise they made when they chose to call themselves the Doctor:

"Never cruel or cowardly. Never give up, never give in."

While this seems like a very Moffat-y line of dialogue, it's actually not new.

This was Moffat's way of referencing Terrance Dicks' description of the character in his 1976 book The Making of Doctor Who:

"He never gives in, and never gives up, however overwhelming the odds against him. The Doctor believes in good and fights evil. Though often caught up in violent situations, he is a man of peace. He is never cruel or cowardly."

Perhaps by this point in the writing process Moff just figured he'd copy from someone else in the class!