Suppose you drew 5 cards and they were all hearts. You might ask “what’s the probability of drawing 5 hearts at random from a 52 card deck?” Well, sure, that’s easy enough to calculate. The probability is 0.000495. Wow! Such a rare event! Must be your lucky day.
But… the reason you asked about 5 hearts is because that’s what you experienced. You peeked at the data, and then asked your question. I am sure if you would have gotten 5 clubs you would have asked about that. Or if you had gotten a straight.
So, one way to rephrase the question is as follows: “what’s the probability of drawing 5 cards at random from a 52 card deck that, upon viewing these cards, would have gotten my attention and prompted me to ask a question about probability?” I’m confident that any flush or straight would have gotten your attention, and four of a kind as well. So let’s stick with those. The probability of drawing either a flush, a straight or four of a kind is 0.006. While this is still a very rare event, it’s about 10 times higher than that of getting 5 hearts.
I have heard it argued that
the probability that you are aware right now, when your existence could have ended billions of years ago, or could have come into being billions of years in the future – this probability is so small, so insignificant, that it is practically non-existent… this could only mean the existence of God.
However, the probability calculation is incorrect. Let’s define the event A as follows:
A: I exist now out of all of the possible times I could have existed
The argument is that P(A) is essentially 0. However, the argument ignores the fact that I already do exist right now, which is why I am asking the question. I am asking a question based on data that I have already seen. We have to condition on that data. Therefore, let’s define the event B as:
B: I exist right now
What we are interested in is not P(A), but P(A|B). We have to condition on B, because B is the reason we are asking the question. It’s the data we peeked at. Well, it turns out that P(A|B)=1. Thus, it’s not a rare event and certainly cannot be an argument in favor of any religious beliefs.
The self-indication assumption (SIA) is
Given the fact that you exist, you should (other things equal) favor hypotheses according to which many observers exist over hypotheses on which few observers exist.
Katja Grace presents a simple example of SIA:
For instance if you were born of an experiment where the flip of a fair coin determined whether one (tails) or two (heads) people were created, and all you know is that and that you exist, SIA says heads was twice as likely as tails.
For simplicity, let’s assume this action only happened once (one coin flip). Thus, there is only one or two people in the world, depending on whether it was tails or heads. Let’s assume if we are in two person world, we don’t see the other person.
Let’s define the event A as:
event A: the coin came up tails (i.e., the one person world)
SIA reasoning is that since there were 3 possible people including myself, and I was selected, the probability that I’m on 1 person world is 1/3.
The flaw here is the focus on me existing. I already exist, so it’s cheating to write questions about me existing after seeing that I exist. Like the card player who formed the hypothesis after seeing the cards, we’re asking the wrong question.
Instead, let’s define the event B as:
event B: at least one person exists
Event B is what we really want to condition on. We want to condition on an arbitrary person existing — there is nothing special about me in this scenario (unless you cheat and use the data you peeked at). Well, in either of these two worlds (heads or tails) there will exist someone that is wondering which world they are in. So, the fact that there is someone wondering which world they are in tells us no information about which world we are in. That is, P(A|B)=P(A)=1/2.
So, in my opinion SIA is wrong. The fact that I exist tells me nothing about the number of observers.
The doomsday argument seems flawed for the same reason. It basically says that the fact that you exist is evidence that the race will die out soon.
It’s correct that if you could randomly draw a human from the N that will exist, you will probably pick one that is towards the tail of the distribution (when the population is greatest). However, we cannot think of ourselves as a random draw. It’s peeking at the data. We already exist.
It should be obvious the argument is flawed based on the fact it always comes to the same conclusion. Suppose, for example, the total number of humans to ever exist (past and future) will total N. Every human, numbers 1, 2, … , N, will at some point exist, and could wonder if humans will face extinction soon. So, if we condition on the fact that right now there is at least one human asking that question, we have no information about whether that human is close to number N. All information about possible extinction would have to rely on other sources.
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