DOCTOR: Hmm, there it is again. That itch. Go down, go down, go down, go down, go down.

IDA: The urge to jump. Do you know where it comes from, that sensation? Genetic heritage. Ever since we were primates in the trees. It's our body's way of testing us. Calculating whether or not we can reach the next branch.

DOCTOR: No, that's not it. That's too kind. It's not the urge to jump. It's deeper than that. It's the urge to fall.