Do you ever wonder why you’re you rather than somebody else? Maybe your best friend, maybe that person who seems to have it all that you’re painfully envious of? Maybe anybody and everybody? To answer that question philosophically, let me turn it around and say, how do you know you’re not?
For all you know, you could be. Maybe you’re just not aware of being any other given person from the vantage point you call your own mind/body/central nervous system. And likewise, from their vantage point, they’re not aware of being you.
Consider if you cloned yourself, or, say, if you had multiple selves in parallel universes (which you probably do—Many Worlds Theory is the interpretation of quantum mechanics that’s most popular among physicists because it’s the most sensical one), but each instance of you is not aware of being each other instance of you, because your bodies/sensory organs/neural networks, which house your experiences, are physically isolated from each other.
Sure, in the case of the clone or the parallel selves, you happen to more or less share an identity with them (such as your name, memories, etc.), but similarity of identity exists on a spectrum of possibilities, and any place you’d draw the line between an identity being “you” versus “not you” is arbitrary, so the absolute truth is probably that there is no line.
And, to circle back to the beginning of this essay, you might be incline to wonder, why weren’t you born as so and so—a completely different person who doesn’t even share an identity with you? And, according to the above reasoning about the relative isolation of bodies—which house your experience, memories, and everything else that gives you your sense of identity and makes you feel distinct—for all you know, you could have been born as them, or even as everyone, in addition to having been born as you, while not realizing it.
And, even more importantly, what could be the necessary actual, measurable/experiential difference between that being the case and that not being the case? If you can’t pin it down to any necessary experiential difference, how can you say there’s any logical difference between the two scenarios? And if you can’t say that, it’s necessarily just as true that you are everyone else as it is that you aren’t.