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.
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.