William M. Briggs

Statistician to the Stars!

The Probability Of Your Existence

Typical college student.

Typical college student.

To the nearest order of magnitude, there have existed, including us, about 100 billion human beings. This proves what was always, until our day, quite obvious: that people are good at breeding. (Or were, before discovering contraception and abortion, of course.)

That’s a lot of people in a relatively short period of time, geologically speaking. Humans have only existed some few hundred thousand years.

According to Harvard Law blogger Ali Binazir, author of Awaken Your Genius, you, dear reader, only have a “1 in 102,685,000” chance of existing. Pretty small, no?

But since there’s been so many of us, each one of us being next to impossible, something has gone wrong in the calculations or our thinking.

Nothing better illustrates the truth that all probability is conditional than Binazir’s calculation. It’s sort of a Drake equation for individual human existence. The Drake equation is a probability-like calculation which gives the chances of other life forms like us, i.e. rational, existing on other planets. It is notorious in its liquidity: it won’t stay solid. Binzair’s calculations suffer from the same flaws: he’s unable to keep straight which side of the equation he’s on.

Given you’re reading this post, the probability you exist is 1, i.e. it is certain. That’s because there must be a you to read. But Binazir tells us that the probability you exist is a number nearly equivalent to 0, the other end of the probability spectrum. Both probabilities can be right, since all probabilities depend on the premises used. Depends on the meaning of “to exist”, also. Does it mean “being made” or “alive and functioning now”?

Alive and functioning now is easy: you exist certainly. But how did you get made? Well, mom met dad and they did what came naturally. Out came you. You, logic insists, could not be other than you. This is necessary to emphasize.

The probability your parents had any kids is also 1, because you are one of them. But the probability that two mated humans reproduce is different. That probability doesn’t exist because no premises have been specified. One possible premise: both over 80 years old? Another: one is sterile, by natural or artificial means. Another: one is forever on the road. And on and on.

Now each time the mates attempt to reproduce, either a baby is produced or it isn’t, a tautology. In either case, something caused the baby to be made or to stop it from being made. We (typically) won’t be in the position of knowing these exact causes, so we say the creation or lack of it is “random”, which is just another label of our ignorance of the causes. But if we assume these causes are “regular”, and are careful to specify the kind of mates to which they apply (say, 20-30 year-olds, of this and such background, etc.), then we can use probability to express our incomplete knowledge and make reasonable predictions over groups of mates.

In any case, we are not asking what is the probability your parents had kids, because they obviously did. We asking the probability of you yourself coming into being. Assuming each of your mom’s and dad’s gametes were unique (are male gametes like snowflakes? has anybody checked? I ask in ignorance), then each mating could make a different baby. But each mating will have a list of causes which either make or stop the baby. So again we’re not talking about what causes the individual gametes to meet.

What premises to use? Any individual attempt? Thus the number of male gametes and a fixed female gamete. In ignorance of the causes, and assuming conception, there are n male gametes and one female, thus the probability is 1/n that this male gamete meets this fixed female one. Is that newly created baby you or somebody else? Assuming no conception, then no you and no anybody either. What is the probability of no conception? Well, what premises do we assume?

You can see this is getting nowhere slow.

The problem is in the question. What is the probability you exist? This is not what is the probability somebody else exists, or the probability somebody in your place exists. It isn’t even the probability some other human exists—which is one by the plain meaning of the question. Why? Because people exist.

You exist, too. Therefore it must be the probability you exist is 1. That you might not have existed is a counterfactual question, which is perfectly comprehensible as long as we supply the premises under which you would not have existed. Say, you imagine your dad got called away on a business trip during the time in which your parents conceived you. Or that your dad never met your mom. Then, given those premises, the probability would not have existed is also 1, because there would have been no causal way of you coming into existence.

Face it. Probability is never as easy as you hope it is.

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Thanks to The Carolina Cowbody for suggesting this topic.

16 Comments

  1. Assuming each of your mom’s and dad’s gametes were unique (are male gametes like snowflakes? has anybody checked? I ask in ignorance)

    Very probably (heh) yes. The process of meiosis https://en.wikipedia.org/wiki/Meiosis includes a step called “crossing over” https://en.wikipedia.org/wiki/Chromosomal_crossover where the chromosomes exchange genetic material and making each eventual gamete unique. The process essentially mixes up the DNA from both parents of the individual producing the gametes which has the benefit of passing genetic diversity (a good thing) to the next generation.

  2. Shouldn’t the question be what is the probability of predicting correctly that a specific person will exist? Which, too, depends on the details of the specification. Merely male, ~ 0.5. Male American, ~ 0.025. Male American wearing a hat…

  3. You did a much better job than I.

  4. I think; therefore I exist. Paraphrase of Rene Descartes.

  5. Some of this must be sinking in. I felt good when I saw the title of the post I thought the probability of me existing is 1. Now, back to the weekly humbling that is me trying to understand the Sunday posts on Aquinas. At least that’s what’s happening now. I mean right now.

  6. and now the probability of the universe…

  7. Given all the stuff there is in the universe, and given that we are made of all the same stuff as the rest of it (proof you don’t need God to make us, as we’re not made of anything that isn’t around us), I’d say we’re not all that surprising. I’m sure there are lots of weird critters around the universe. That we have certain features, that would narrow things, but again, I’m sure there are all sorts of weird features out there too.

    JMJ

  8. “Given all the stuff there is in the universe, and given that we are made of all the same stuff as the rest of it (proof you don’t need God to make us, as we’re not made of anything that isn’t around us)”

    One of the very first principles discovered by science is that life does not arise spontaneously from non-living matter. So, unless you are arguing that we aren’t really alive, the mere fact that we are made of the same materials as the rest of the universe is not proof that we don’t need God to make us.

  9. “One of the very first principles discovered by science is that life does not arise spontaneously from non-living matter.”

    No. There is no such principle. Silliness.

    JMJ

  10. Sander van der Wal

    July 7, 2014 at 11:44 pm

    Most of the universe is made of spacetime. We are not made of spacetime.

    Almost all particles in the universe are Dark Matter particles. We are not made of Dark Matter particles.

    There are a few fermions (quarks and electrons) around. We are made of fermions.

    So we are made of the rarest stuff in the universe.

  11. “We are stardust, we are golden, we are billion year old carbon,
    And we got to get ourselves back to the garden.”
    – CSN&Y

  12. Probability that the “typical college student” is a Sleeping Genius who just needs to be Awakened: 1

  13. Alan McIntire

    July 8, 2014 at 11:53 am

    “Ray
    7 July 2014 at 1:11 pm

    I think; therefore I exist. Paraphrase of Rene Descartes”
    That reminds me of a “Raymond Smullyan” joke:
    “I think, therefore I am– could be– unless I’m really someone else who only THINKS they’re me”

  14. Just to be clear are you asking us to evaluate :

    1) the probabilty that we (I) has spontaneously arisen

    2) the probability that the two meiotic sets that make up my DNA might ever come together

    3) given that my existence is predicated on being born from parents, the probability that my DNA will be generated from the available meiotic combinations crossed with the risk of mutation and copying error

    .??

  15. If there have been 100 billion human beings, that means only 93% of all humans have died. In other words, the theory that “all men are mortal” is no longer established to within a 95% confidence interval.

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