Let’s say one of your kidneys went bad and you were lucky enough to get a donated one. You would still be you, right? Of course. Well, what if you got 2 kidneys? Sure. What if you also got new sets of lungs, eyes, and ears? That starts to get weird. But let’s keep going–suppose a surgeon took one cell from your brain and swapped it with one cell from your friend’s brain. You would be the same person, right? Sure. And so would your friend. But what if the surgeon kept swapping cells until you and your friend had exchanged exactly 50% of all of your brain cells? Would you still be you? And if so, which body would be yours? And how would you know? Most importantly, to what being does “you” now refer?