DOCTOR: Out there in that city beneath us, that is the essence of life. It isn't just about the beat of our hearts, the breaths we take: it's our thoughts, our memories, our experiences - a myriad of emotions, family, friends, who we love, who we hate. The devolver takes all that away. To stop feeling and experiencing all these things is death, and if everyone around you has been destroyed then even the memory of your ever having lived ceases to exist - all that's left behind is nothing. Is that what you want?