“[Think] of an experience from your childhood. Something you remember clearly, something you can see, feel, maybe even smell, as if you were really there. After all you really were there at the time, weren't you? How else could you remember it? But here is the bombshell: you weren't there. Not a single atom that is in your body today was there when that event took place. Every bit of you has been replaced many times over (which is why you eat, of course). You are not even the same shape as you were then. The point is that you are like a cloud: something that persists over long periods, while simultaneously being in flux. Matter flows from place to place and momentarily comes together to be you. Whatever you are, therefore, you are not the stuff of which you are made. If that does not make the hair stand up on the back of your neck, read it again until it does, because it is important.”