The TARDIS responds to his feelings with her own comfort and the reassurance that everything will get better with time. But she knows how much time that will take, and how very much worse it's going to get first. She has to take care to hide her own feelings of sadness at remembering Donna's loss, and the fear at the thought of what comes afterwards, of his descent into isolation that ended on Mars.

But she knows what he needs from her right now, and that at least she can give him. "Donna survives," she assures him firmly. "When the time comes, you will know what to do to save her."