"Keep him in your mind. If you forget, you'll lose him forever." - Eleven

Isn't that how it always works?

"People say you die twice: once when you physically die, and once when someone says your name for the last time."

When the TARDIS shook, Amy looked up, and the Doctor could see a faint sort of puzzlement in her features.
"Forget who?" She asked, and he felt his two hearts shatter. Sometimes, she would look off into the distance, and she would see something, anything- a blue vest, a figure of a Roman centurion, a shock of dirty blonde hair- and tears would start falling down her face.
"I don't even know why I'm crying" she would say, and then wipe her tears away as if nothing had happened.

"People fall out of the world sometimes, but they- they always leave traces, little things you can't quite account for: faces in photographs, luggage, half-eaten meals...rings. Nothing is ever forgotten, not completely, and, if something can be remembered, it can come back."

The human heart is a TARDIS, and deep in the bowels of the beloved ship, rooms upon rooms upon rooms of people, faces...small slices of time that were meant to make a home in there.