10 Doctor Who Mysteries You Didn't Know Were Answered
6. How Can The Doctor Remember Heaven Sent?
In Heaven Sent, the Doctor spends 4.5 billion years inside the confession dial. Each time he dies, he powers up a machine that beams in a brand-new Doctor. So surely, the final Doctor that breaks through the wall cannot possibly remember every moment of those 4.5 billion years.
So why does he say that he can "remember it all" at the end of each loop? Massive Time Lord brain or not, that's a lot of memories to keep in one head.
Steven Moffat discussed this in Doctor Who Magazine #495, in answer to a fan's question. Moffat's belief is that the Doctor essentially creates those memories for himself. When he understands that he must break through the wall and when he notices all those skulls in the water, he realises that he's been doing this for a very long time, and the weight of all those centuries becomes his memory.
He doesn't necessarily remember every single detail, but each version of the Doctor realises that they've been there for thousands, millions, billions of years each time they get to the diamond wall.
As an extra flourish, Moffat also suggests that the Doctor remembers because the confession dial won't let him forget, saying that:
"In a world designed to suck your bad dreams from your mind and feed them back to you, isn't it possible that his worst day - the one he's living right now, again and again - is hanging in the air around him?"
Oh Moffat, you clever sod.