Imagine the following alternative history of the world: Things are qualitatively just as they actually are. There is no difference in anything like the shape, size, or mass of objects. There is no difference in the number of entities. Even so, there is a non-qualitative difference and it concerns you in particular. According to this alternative history, you fail to exist. In your place, there is a distinct individual, Double. Double has all the qualitative properties, whether mental or physical, you actually have, but, despite all these similarities, you and Double are distinct individuals. So, according to this alternative history, you do not exist.