The anthropic principle is contingent on no additional information. For example, if sentient life exists elsewhere in the universe, your odds of being a human are vanishingly small. This would suggest sentient life does not exist elsewhere in the universe. However, given that there appears to be nothing so special about earth that it wouldn’t reoccur many times among trillions and trillions of stars, we can still conclude that sentient life does likely exist elsewhere in the universe.
Similarly, in this context, the fact that animals have brains that are relatively similar to ours itself gives you evidence with which to refine the anthropic argument. As you said, that hard line between experience-having and not-experience having would be weird. Thus, evidence from the observed universe trumps, or at least significantly adjusts, the anthropic argument.
It seems to take very tiny pieces of evidence to destroy a lot of anthropic reasoning, which is why, as much as I’d enjoy me some fillet-’o-chimp, I don’t generally trust anthropic reasoning as a stopping point; we can often improve on it with available information.
For example, if sentient life exists elsewhere in the universe, your odds of being a human are vanishingly small. This would suggest sentient life does not exist elsewhere in the universe.
That’s not how the anthropic principle works.
The anthropic principle lets you compute the posterior probability of some value V of the world, given an observable W. The observable can be the number of humans who have lived so far, and the value V can be the number of humans who will ever live. The probability of a V where 100W < V is smaller than the probability of a V only a few times larger than W.
It’s unclear if you get to count transhumans and AIs in V, which is the same problem Yvain is raising here about whether to include bats and ants in the distribution.
You can’t conclude that there aren’t other planets with life because you ended up here, because the probability of different values of V doesn’t depend on the observable W. There’s no obvious reason why P(there are 9999 other planets with life | I’m on this planet here with life) / P(there are 9999 other planets with life) would be different than
P(there are 0 other planets with life | I’m on this planet with life) / P(there are 0 other planets with life).
(I divided by the priors to show that the anthropic principle takes effect only in the conditional probability; having a different prior probability is not an anthropic effect.)
Disclaimer: I’m a little drunk.
I’m troubled now that this formulation doesn’t seem to work, because it relies on saying “P(fraction of all humans who have lived so far is < X)”. It doesn’t work if you replace the “<” with an “=”. But the observable has an “=”.
BTW, outside transhumanist circles, the anthropic principle is usually used to justify having a universe fine-tuned for life, not to figure out where you stand in time, or whether life will go extinct.
The anthropic principle lets you compute the posterior probability of some value V of the world, given an observable W. The observable can be the number of humans who have lived so far, and the value V can be the number of humans who will ever live. The probability of a V where 100W < V is smaller than the probability of a V only a few times larger than W.
This argument could have been made by any intelligent being, at any point in history, and up to 1500AD or so we have strong evidence that it was wrong every time.
If this is the main use of the anthropic argument, then I think we have to conclude that the anthropic argument is wrong and useless.
I would be interested in hearing examples of applications of the anthropic argument which are not vulnerable to the “depending on your reference class you get results that are either completely bogus or, in the best case, unverifiable” counterargument.
(I don’t mean to pick on you specifically; lots of commentors seem to have made the above claim, and yours was simply the most well-explained.)
This argument could have been made by any intelligent being, at any point in history, and up to 1500AD or so we have strong evidence that it was wrong every time. If this is the main use of the anthropic argument, then I think we have to conclude that the anthropic argument is wrong and useless.
First, “the anthropic argument” usually refers to the argument that the universe has physical constants and other initial conditions favorable to life, because if it didn’t, we wouldn’t be here arguing about it.
Second, what you say is true, but someone making the argument already knows this. The anthropic argument says that “people before 1500AD” is clearly not a random sample, but “you, the person now conscious” is a random sample drawn from all of history, although a sample of very small size.
You can dismiss anthropic reasoning along those lines for having too small a sample size, without dismissing the anthropic argument.
Thank you for saying this. I agree. Since at least the time I made this comment, I have tentatively concluded that anthropic reasoning is useless (i.e. necessarily uninformative), and am looking for a counterexample.
The anthropic principle is contingent on no additional information. For example, if sentient life exists elsewhere in the universe, your odds of being a human are vanishingly small.
True, assuming sentient life is common enough.
This would suggest sentient life does not exist elsewhere in the universe.
Not true. This is like saying that if you roll a million sided die and get 362,853 then the die must have been fixed because the chance of getting 362,853 is 1-in-a-million!
if you roll a million sided die and get 362,853 then the die must have been fixed because the chance of getting 362,853 is 1-in-a-million!
Were that appropriate, the same mechanism would also defeat the reasoning in this post. While I agree with your ultimate conclusion, using solely the anthropic principle and no additional information, I believe you are compelled to conclude extraterrestrial life does not exist.
Were that appropriate, the same mechanism would also defeat the reasoning in this post.
I disagree. There is a natural category (sentience, reflectivity, etc.) that picks out humans over other Earthly animals and leads to a more-than-max-entropy prior for humans being more anthropically special*; this is not the case for either 362,853 or Earth.
* If you accept anthropic reasoning at all, that is. I’m sort of playing devil’s advocate in this comment; this post mostly just pushes me further towards biting the bullet of UDT/collapsing epistemology to decision theory.
The anthropic principle is contingent on no additional information. For example, if sentient life exists elsewhere in the universe, your odds of being a human are vanishingly small. This would suggest sentient life does not exist elsewhere in the universe. However, given that there appears to be nothing so special about earth that it wouldn’t reoccur many times among trillions and trillions of stars, we can still conclude that sentient life does likely exist elsewhere in the universe.
With an acknowledgement that on topics of this difficulty I don’t expect to be right a supermajority of the time I have to disagree both on what “I am human” tells me about other beings and on what extra information tells me.
Given no additional information, noticing that I am a human increases the probability that there is sentient life elsewhere in the universe (it at least shows that sentient life is possible). It is a mistake to draw any conclusions from p(a randomly chosen sentient being is a human | there are sentient beings elsewhere in the universe). If both you and aliens exist then you and aliens exist. Knowing that you happen to be you instead of an alien isn’t particularly significant.
As for extra information… well, the fact that we can’t see any evidence of interstellar civilisations eating stars or otherwise messing up the place does provide weak-to-moderate evidence that intelligent life is hard to come by depending on how likely it is for intelligent life to progress that far. In that case anthropic reasoning would help explain how we could come to exist given that life was improbable. We would be an unimaginably improbable freak and all other similarly improbable freaks would be off in other Everett branches.
If I am using the anthropic principle and the observation that I am human, these together provide very strong evidence that we are in either world one or world two, with a slightly stronger nudge towards world one. Where we end up after this observation depends on our priors. I agree fully that making additional inferences, such as the probability of other sentient beings increasing due to our own existence, or when we look at the size of the universe, the odds of being alone decrease, affects the end probability.
The inference I described may be unduly restricted, but that is my exact point. The original post made an anthropic inference in isolation—it simply used the fact that there are more animals than humans, and the author is a human, to infer that animals do not have experiences. The form of the argument would not have changed significantly were it used to argue that rocks lack experience. Thus, while the argument is legitimate, it is easily overwhelmed by additional evidence, such as the fact that humans and animals have somewhat similar brains. That was my point: the anthropic principle is easily swamped by additional evidence (as in the ET issue) and so is being overextended here.
You’re saying, “I rolled a die. The die came up 1. Therefore, this die probably has a small number of sides.”
But “human” is just “what we are”. Humans are not “species number 1”. So your logic is really like saying, “I rolled a die. The die landed with some symbol on top. Therefore, it probably has a small number of sides.”
If the die is small enough for you to hold in one hand, and the symbol covers only side yet is large enough to easily read with typical human visual acuity, based on the laws of geometry it would be safe to assume that the die has fewer than about 100 sides, yeah.
I think this part of the analogy equates to our ability to observe the rest of the universe over billion-year time frames and its apparent lack of alien life forms.
The Doomsday argument is part observational, after all.
If the various species of ET are such that no particular species makes up the bulk of sentient life, then there’s no reason to be surprised at belonging to one species rather than another. You had to be some species, and human is just as likely as klingon or wookie.
If I am using the anthropic principle and the observation that I am human, these together provide very strong evidence that we are in either world one or world two, with a slightly stronger nudge towards world one.
And here is where we are in simple disagreement. I say that knowing that I am human tells me very little about the configuration of matter in a different galaxy. Things that it does not tell me include, but are not limited to, “is the matter arranged in the form of a childlike humanoid, maybe green or grey. Probably with a big head and that can do complex thinking?”
I claim (and, again, it is a complex topic so I wouldn’t bet on myself at odds of more than one gives you, say, 20) that this argument isn’t weak evidence that is easily overwhelmed. It is not evidence at all.
The anthropic principle is contingent on no additional information. For example, if sentient life exists elsewhere in the universe, your odds of being a human are vanishingly small. This would suggest sentient life does not exist elsewhere in the universe. However, given that there appears to be nothing so special about earth that it wouldn’t reoccur many times among trillions and trillions of stars, we can still conclude that sentient life does likely exist elsewhere in the universe.
Similarly, in this context, the fact that animals have brains that are relatively similar to ours itself gives you evidence with which to refine the anthropic argument. As you said, that hard line between experience-having and not-experience having would be weird. Thus, evidence from the observed universe trumps, or at least significantly adjusts, the anthropic argument.
It seems to take very tiny pieces of evidence to destroy a lot of anthropic reasoning, which is why, as much as I’d enjoy me some fillet-’o-chimp, I don’t generally trust anthropic reasoning as a stopping point; we can often improve on it with available information.
That’s not how the anthropic principle works.
The anthropic principle lets you compute the posterior probability of some value V of the world, given an observable W. The observable can be the number of humans who have lived so far, and the value V can be the number of humans who will ever live. The probability of a V where 100W < V is smaller than the probability of a V only a few times larger than W.
It’s unclear if you get to count transhumans and AIs in V, which is the same problem Yvain is raising here about whether to include bats and ants in the distribution.
You can’t conclude that there aren’t other planets with life because you ended up here, because the probability of different values of V doesn’t depend on the observable W. There’s no obvious reason why P(there are 9999 other planets with life | I’m on this planet here with life) / P(there are 9999 other planets with life) would be different than P(there are 0 other planets with life | I’m on this planet with life) / P(there are 0 other planets with life).
(I divided by the priors to show that the anthropic principle takes effect only in the conditional probability; having a different prior probability is not an anthropic effect.)
Disclaimer: I’m a little drunk.
I’m troubled now that this formulation doesn’t seem to work, because it relies on saying “P(fraction of all humans who have lived so far is < X)”. It doesn’t work if you replace the “<” with an “=”. But the observable has an “=”.
BTW, outside transhumanist circles, the anthropic principle is usually used to justify having a universe fine-tuned for life, not to figure out where you stand in time, or whether life will go extinct.
This argument could have been made by any intelligent being, at any point in history, and up to 1500AD or so we have strong evidence that it was wrong every time. If this is the main use of the anthropic argument, then I think we have to conclude that the anthropic argument is wrong and useless.
I would be interested in hearing examples of applications of the anthropic argument which are not vulnerable to the “depending on your reference class you get results that are either completely bogus or, in the best case, unverifiable” counterargument.
(I don’t mean to pick on you specifically; lots of commentors seem to have made the above claim, and yours was simply the most well-explained.)
First, “the anthropic argument” usually refers to the argument that the universe has physical constants and other initial conditions favorable to life, because if it didn’t, we wouldn’t be here arguing about it.
Second, what you say is true, but someone making the argument already knows this. The anthropic argument says that “people before 1500AD” is clearly not a random sample, but “you, the person now conscious” is a random sample drawn from all of history, although a sample of very small size.
You can dismiss anthropic reasoning along those lines for having too small a sample size, without dismissing the anthropic argument.
Thank you for saying this. I agree. Since at least the time I made this comment, I have tentatively concluded that anthropic reasoning is useless (i.e. necessarily uninformative), and am looking for a counterexample.
Best time to do anthropic reasoning. Save the sane reasoning for when you’re sober! ;)
True, assuming sentient life is common enough.
Not true. This is like saying that if you roll a million sided die and get 362,853 then the die must have been fixed because the chance of getting 362,853 is 1-in-a-million!
Were that appropriate, the same mechanism would also defeat the reasoning in this post. While I agree with your ultimate conclusion, using solely the anthropic principle and no additional information, I believe you are compelled to conclude extraterrestrial life does not exist.
I disagree. There is a natural category (sentience, reflectivity, etc.) that picks out humans over other Earthly animals and leads to a more-than-max-entropy prior for humans being more anthropically special*; this is not the case for either 362,853 or Earth.
* If you accept anthropic reasoning at all, that is. I’m sort of playing devil’s advocate in this comment; this post mostly just pushes me further towards biting the bullet of UDT/collapsing epistemology to decision theory.
With an acknowledgement that on topics of this difficulty I don’t expect to be right a supermajority of the time I have to disagree both on what “I am human” tells me about other beings and on what extra information tells me.
Given no additional information, noticing that I am a human increases the probability that there is sentient life elsewhere in the universe (it at least shows that sentient life is possible). It is a mistake to draw any conclusions from p(a randomly chosen sentient being is a human | there are sentient beings elsewhere in the universe). If both you and aliens exist then you and aliens exist. Knowing that you happen to be you instead of an alien isn’t particularly significant.
As for extra information… well, the fact that we can’t see any evidence of interstellar civilisations eating stars or otherwise messing up the place does provide weak-to-moderate evidence that intelligent life is hard to come by depending on how likely it is for intelligent life to progress that far. In that case anthropic reasoning would help explain how we could come to exist given that life was improbable. We would be an unimaginably improbable freak and all other similarly improbable freaks would be off in other Everett branches.
Assume three possible worlds, for simplicity:
A: 1 billion humans. No ETs.
B: 1 billion humans, 1 million ETs
C: 1 billion humans, 1 billion billion billion ETs.
If I am using the anthropic principle and the observation that I am human, these together provide very strong evidence that we are in either world one or world two, with a slightly stronger nudge towards world one. Where we end up after this observation depends on our priors. I agree fully that making additional inferences, such as the probability of other sentient beings increasing due to our own existence, or when we look at the size of the universe, the odds of being alone decrease, affects the end probability.
The inference I described may be unduly restricted, but that is my exact point. The original post made an anthropic inference in isolation—it simply used the fact that there are more animals than humans, and the author is a human, to infer that animals do not have experiences. The form of the argument would not have changed significantly were it used to argue that rocks lack experience. Thus, while the argument is legitimate, it is easily overwhelmed by additional evidence, such as the fact that humans and animals have somewhat similar brains. That was my point: the anthropic principle is easily swamped by additional evidence (as in the ET issue) and so is being overextended here.
You’re saying, “I rolled a die. The die came up 1. Therefore, this die probably has a small number of sides.”
But “human” is just “what we are”. Humans are not “species number 1”. So your logic is really like saying, “I rolled a die. The die landed with some symbol on top. Therefore, it probably has a small number of sides.”
If the die is small enough for you to hold in one hand, and the symbol covers only side yet is large enough to easily read with typical human visual acuity, based on the laws of geometry it would be safe to assume that the die has fewer than about 100 sides, yeah.
I think this part of the analogy equates to our ability to observe the rest of the universe over billion-year time frames and its apparent lack of alien life forms.
The Doomsday argument is part observational, after all.
If the various species of ET are such that no particular species makes up the bulk of sentient life, then there’s no reason to be surprised at belonging to one species rather than another. You had to be some species, and human is just as likely as klingon or wookie.
And here is where we are in simple disagreement. I say that knowing that I am human tells me very little about the configuration of matter in a different galaxy. Things that it does not tell me include, but are not limited to, “is the matter arranged in the form of a childlike humanoid, maybe green or grey. Probably with a big head and that can do complex thinking?”
I claim (and, again, it is a complex topic so I wouldn’t bet on myself at odds of more than one gives you, say, 20) that this argument isn’t weak evidence that is easily overwhelmed. It is not evidence at all.