"According to K'anpo Rinpoche, each time the Doctor regenerates his cells are replaced and his brain structure is rearranged. Consequently, each Doctor has been quite unique. First was a crotchedy old man-a white-haired grandfather giving those young whipper-snappers "what for." By contrast, the Tenth was an exuberant young man-a great-haired lover, brash and impetuous. In between, he's been a clown (Sixth), a pleasant uncle (Third), a cricket player (Fifth), a scatterbrained jelly-baby-offering comedian (Fourth), and Moe from the Three Stooges (Second). But does regeneration make the Doctor, literally, a different person? K'anpo Rinpoche suggests the difference is merely metaphorical. He'll behave and look different, sure, but he won't be literally a different person-like twins are literally different persons. The Doctor still is the Doctor, regeneration after regeneration. If we saw the First and Tenth Doctor together, however, there'd be nothing to make us think they are, literally, the same person. They're more different than they're the same-more different then you and the person across the room. You and he or she aren't the same person, right? So how could each regeneration of the Doctor be, literally, the same person?"