"What we're asking is how the "replacements" could all be one in the same person. Non-Who fans might wonder, "Who cares?" But think about yourself at age ten. You acted differently, had a different worldview, a different personality, and different beliefs. As you learned, your brain structure was rearranged. In fact, gradually, your body completely replaces all of its cells using material and energy you ingested. Just like the Doctor, you regenerate; and you at ten is less like you now than that person across the room. How could you at ten and you now possibly be the same person? So our question about the Doctor's identity is very relevant. By answering it we can answer an important question about ourselves: How is it that we retain our identity over time?"