"[T]he differences between minds would then be like the differences between languages, or styles of music or art—inexhaustible in the limit, but approachable to any degree of approximation you like. But the difference between having a mind and not having a mind at all—between being something with its own subjective point of view and being something that is all outside and no inside, like a rock or a discarded sliver of fingernail—is apparently an all-or-nothing difference."