Toward the end of his book “Why Does the World Exist?” Jim Holt poses a brain-boggler about the self.

“If you happen to be one of a pair of identical twins, try this thought experiment. Imagine that the zygote that split apart shortly after fertilization to produce you and your twin had instead remained a single clump of cells. Would the unique baby born to your parents nine months later have been you? Your twin? Neither?”

I’ll pause and clean my desk for a minute while you think about that.

I’m back. (I didn’t make much progress on my desk, actually. If I were to try to clean my desk thoroughly, we’d never get around to answering the question.)

Obviously, the “unique baby” would not be “you,” nor your twin. The answer is “neither.” A  person — the “you” here — is more than a genome. When we talk about “you” we are talking about more than genetics. We’re talking about genetics plus gestational development plus childhood environment and so on. In this thought experiment, you are reflexively imagining yourself as a developed “you,” loaded with memories and personality traits from countless experiences that shaped your personality and sense of self. The zygote is your genetic duplicate in this thought experiment, but it’s not you any more than your identical twin would be you.

And if you get cloned someday, your clone will not be “you.”

What Holt explains in his book is that, in a psychological sense, the “you” evolves over time, and — notwithstanding what the Social Security Administration and other government agencies might claim —  the you of today isn’t exactly the same person as the you of, say, 35 years ago (but maybe, once again, I am just trying to distance myself from my pathetic high school self). In a way, it’s a new you every morning when you wake up. (The counter-argument is that the “you” is simply your brain, and that you stay you even if you have brain surgery or gobble mind-altering pills.)

Lurking within this is the question of why you are you and not someone else. Why of all the 7 billion people in the world, did you turn out to be you? What were the odds of that? There’s a strange crapshoot element to being you, isn’t there? (And what’s it like being, say, George Clooney? Doesn’t he feel a little strange, and a tad embarrassed, being George Clooney? Surely he feels like his existence as the person known as George Clooney is extremely unlikely. One feels a little bit bad for him, because he’s stuck being George Clooney even if he’d rather be just an ordinary shmoe walking around in baggy shorts with little packets of Burpee seeds in his pocket. Squash, pumpkins, beans. He can’t be just a guy! Instead he’s a prisoner of his George Clooneyness.)

Just as you can’t prove that the “present” is real,  it’s hard to prove that you inhabit a special place in the universe that we will call, for convenience, and at the risk of being obscure, “you.” In all the universe, you exist in a single corporeal body and not in any other, and your meat-self includes a highly complex organ in your skull from which consciousness and the “self” somehow emerges. But everyone feels the same way. Consciousness is notoriously hard to nail down, and the “self” is surely, to some degree, a figment, a side-effect of a mechanistic process that enables superior outcomes through temporal flexibility and modeling (tomorrow we will wait in ambush for the herd to come into the gully). So although you feel as though you’re in a privileged position, you have no proof. And no, I’m not accepting any of the forms of ID you are carrying in your pocket. You’re going to have to do a lot better than that, Buster!