DOCTOR: You see? Used the regeneration energy to heal myself, but soon as I was done, I didn't need to change. I didn't want to. Why would I? Look at me. So, to stop the energy going all the way, I siphoned off the rest into a handy bio-matching receptacle, namely my hand. My hand there. My handy spare hand. Remember? Christmas Day, Sycorax. Lost my hand in a sword fight? That's my hand. What do you think?

ROSE: You're still you?

DOCTOR: I'm still me.