Children, at some age, will begin to understand that there is death, the end of a human life (in material form, at least). At about the same time, in my experience, they will begin to speculate about the possibility that they might have been someone else: a child born in China, for instance.
Death eventually loses its fascination, though it may come to mind from time to time as one grows old. (Will I wake up in the morning? Is this the day that my heart stops beating? Will I be able to break my fall when the heart attack happens, or will I just go down hard and die of a fractured skull?)
But after careful reflection, at some age, the question of being been born as someone else is answered in the negative.
For each person, there is only one “I”, the unique “me”. If I hadn’t been born, I wouldn’t be “I” — there wouldn’t be a “me”. I couldn’t have been born as someone else: a child born in China, instance. A child born in China — or at any place and time other than where and when my mother gave birth to me — must be a different “I’. not the one I think of as “me”.
(Inspired by Sir Roger Scruton’s On Human Nature, for which I thank my son.)