Consider a photo of someone you think is you eight years ago. What makes that person you? You might say he she was composed of the same cells as you now. But most of your cells are replaced every seven years. You might instead say you're an organism, a particular human being, and that organisms can survive cell replacement - this oak being the same tree as the sapling I planted last year.

But are you really an entire human being? If surgeons swapped George Bush's brain for yours, surely the Bush look-alike, recovering from the operation in the White House, would be you. Hence it is tempting to say that you are a human brain, not a human being.

But why the brain and not the spleen? Presumably because the brain supports your mental states, eg your hopes, fears, beliefs, values, and memories. But then it looks like it's actually those mental states that count, not the brain supporting them. So the view is that even if the surgeons didn't implant your brain in Bush's skull, but merely scanned it, wiped it, and then imprinted its states on to Bush's pre-wiped brain, the Bush look-alike recovering in the White House would again be you.

But the view faces a problem: what if surgeons imprinted your mental states on two pre-wiped brains: George Bush's and Gordon Brown's? Would you be in the White House or in Downing Street? There's nothing on which to base a sensible choice. Yet one person cannot be in two places at once.

In the end, then, no attempt to make sense of your continued existence over time works. You are not the person who started reading this article.