People are bad at interpreting the Doomsday Argument, because people are bad at dealing with evidence as Bayesian evidence, rather than a direct statement of the correct belief.
The Doomsday Argument is evidence that we should update on. But it is not a direct statement of the correct belief.
A parable:
On a parallel earth, humanity is on the decline. Some disaster has struck, and the once-billions of proud humanity have been reduced to a few scattered thousands. Now the last exiles of civilization hide in sealed habitats that they no longer have the supply chains to repair, and they know that soon enough the end will come for them too. But on the other hand, the philosophers among them remark, at least there’s the Doomsday Argument, which says that on average we should expect to be in the middle of humanity. So if the DA is right, the current crisis is merely a bottleneck in the middle of humanity’s time, and everything will probably work itself out any day now. The last philosopher dies after breathing in contaminated air, with the last words “No! The position I occupy is… very unlikely!”
Moral:
Your eyes and ears also provide you evidence about the expected span of humanity.
But isn’t the point of the Doomsday Argument that we’ll need very very VERY strong evidence to the contrary to have any confidence that we’re not doomed? Perhaps we should focus on drastically controlling future population growth to better our chances of prolonged survival?
To believe that you’re a one in a million case (e.g. in the first or last millionth of all humans), you need 20 bits of information (because 2^20 is about 1000000).
So on the one hand, 20 bits can be hard to get if the topic is hard to get reliable information about. But we regularly get more than 20 bits of information about all sorts of questions (reading this comment has probably given you more than 20 bits of information). So how hard this should “feel” depends heavily on how well we can translate our observational data into information about the future of humanity.
Extra note: In the case that there are an infinite number of humans, this uniform prior actually breaks down (or else naively you’d think you have a 0.0% chance of being anyone at all), so there can be a finite contribution from the possibility that there are infinite people.
People are bad at interpreting the Doomsday Argument, because people are bad at dealing with evidence as Bayesian evidence, rather than a direct statement of the correct belief.
The Doomsday Argument is evidence that we should update on. But it is not a direct statement of the correct belief.
A parable:
On a parallel earth, humanity is on the decline. Some disaster has struck, and the once-billions of proud humanity have been reduced to a few scattered thousands. Now the last exiles of civilization hide in sealed habitats that they no longer have the supply chains to repair, and they know that soon enough the end will come for them too. But on the other hand, the philosophers among them remark, at least there’s the Doomsday Argument, which says that on average we should expect to be in the middle of humanity. So if the DA is right, the current crisis is merely a bottleneck in the middle of humanity’s time, and everything will probably work itself out any day now. The last philosopher dies after breathing in contaminated air, with the last words “No! The position I occupy is… very unlikely!”
Moral:
Your eyes and ears also provide you evidence about the expected span of humanity.
But isn’t the point of the Doomsday Argument that we’ll need very very VERY strong evidence to the contrary to have any confidence that we’re not doomed? Perhaps we should focus on drastically controlling future population growth to better our chances of prolonged survival?
To believe that you’re a one in a million case (e.g. in the first or last millionth of all humans), you need 20 bits of information (because 2^20 is about 1000000).
So on the one hand, 20 bits can be hard to get if the topic is hard to get reliable information about. But we regularly get more than 20 bits of information about all sorts of questions (reading this comment has probably given you more than 20 bits of information). So how hard this should “feel” depends heavily on how well we can translate our observational data into information about the future of humanity.
Extra note: In the case that there are an infinite number of humans, this uniform prior actually breaks down (or else naively you’d think you have a 0.0% chance of being anyone at all), so there can be a finite contribution from the possibility that there are infinite people.