Each of us is an utterly unique event. This follows from the two things that make us what we are: genetics and history.
Unique Inheritance
Genetically, every physical trait we have is determined some of the tens of thousands of genes our DNA. Gross features like our hair and eye color; subtle features like our tendency toward, or resistance to, certain diseases; features obvious from the cradle and features that only manifest late in life: every possible trait with an organic basis is established by our genes.
And, because the physical structure of our brains provides the machinery of thought, many if not most of the major tendencies and capabilities of our minds and personalities are also established by our genes. Whether we are risk-averse or risk-seeking; whether we can remember musical harmonies, or only remember melodies, or are tone-deaf; whether we are good at spatial orientation or get lost in a closet; whether we flourish in company or prefer to brood alone: all these traits are life-defining and all have a genetic basis. (In twin studies, identical twins separated at birth often prove as adults to have near-identical tastes in clothes and food.)
We get this package of traits and possibilities from our parents, of course, but in an amazing way. In our DNA, each gene is present in double form, two copies, one copy on each side of the ladder-shaped DNA molecule. The copies are not identical. We got one copy from Mom and one copy from Dad, and they can differ. There are lots of complex rules about which copy the cell actually uses when it needs to make a protein (sometimes the paternal copy wins, sometimes the maternal one, which is why my balding hair pattern is exactly like my mother's father's) but the basic idea is: your traits are a mix of your parent's traits.
But wait, there's more. When a male's body makes a sperm, or a female's, an egg, something absolutely zany happens. The double strand of DNA is unzipped, with each strand going to make one germ cell (sperm or egg). That makes sense; when an egg and sperm get together, they'll combine their single strands to give the child a set of properly double-stranded DNA, one strand from each parent.
Let's let one letter stand for each gene along your DNA, at this point just M for Mom's and D for Dad's. In this view, your genome is pretty boring, just a series of gene pairs MD-MD-MD-MD...
But wait, there's more: before making two germ cells, the split strands of DNA randomly exchange segments of genetic information (Wikipedia:meiosis). So what you get from your father's DNA is a random mixture of his parent's contributions. And what you get from your mother's DNA, likewise, a random mixture of genes she got from Nana and Fafa.
Now let's represent a genome with pairs from four letters: N for Nana, your maternal granny, F for mom's dad Fafa, M for dad's MomMom and P for dad's PopPop. Each gene copy you got from your mother was randomly an N or an F, and each copy from your father was randomly an M or P, so your genome (or mine) could be: FP-NM-FM-NP-FM-FP-NP-... with each gene one of 4 combinations of your grandparents' genes at that position.
A human genome has at least 20,000 genes. On that basis there are 420000 possible combinations of grandparental genes. That's already an unimaginably huge number but we can make it larger. Let your mother's four grandparents be a, b, c and d. Let father's be A, B, C, D. Now you can represent your genome as aC-cD-aC-bC-cB-bD-... and so on, with each gene being one of 16 possible combinations of great-grandparents' genes. Now there are 1620000 ways to make a genome.
We could continue the math games, adding generations, but after the 6th or 7th (when the genome is a number in 12820000) we have to start thinking about multiple contributions, where we inherit from the same people on both sides of the family (cousins marrying, etc.).
It doesn't matter. There won't be 1620000 humans born in all of time, or 220000 for that matter. Your precise combination of genes, nor mine nor anyone's, has never existed and will never be repeated before the heat death of the universe.
Moment in Time
From the moment of conception a person's genome is in dynamic interaction with its environment. The body grows, changes, struggles, waxes and wanes as the environment lets it; and as soon as birth happens the brain begins to do the same. Every experience changes us. Even identical twins have different life histories, right from infancy: one is fed before the other; one sees a butterfly and the other is napping; one throws a snowball and the other is hit.
We are altered moment by moment by experiences trivial and shattering. Every interaction between our senses and the physical world changes us, as does every interaction with another person—obviously some more than others, but nothing that penetrates consciousness fails to leave a mark however light.
And your life history is different from mine and every other person alive, even people born in the same place, same time, same social class, same ethnicity.
And your history cannot be the same as that of any person born in another place or another moment in history. Sure, when you look at the ocean, you see "the same" ocean as did Whonk, the Neanderthal, and everyone between you and him. But you got to that view wearing shoes... and your perception of what you see is different in a thousand ways from Whonk's, because you know the ocean as the skin on a globe, something traversed by people. Even if you look at the ocean with your identical twin, you can't be certain the pair of you are seeing "the same" ocean—because of the time when you saw an Orca lobbing, and your twin was looking the other way and missed it.
Even if there were another genome like yours to nine places, the person bearing it couldn't be the same as you because that twin couldn't share your identical life history.
Summary
Bottom line: you, me, any human being, is as utterly unique as a phenomenon can be, never to be repeated. Any philosophy must acknowledge and build on this.
Next: we contradict all of the above.
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