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DanielLC comments on [Resolved] Is the SIA doomsday argument wrong? - Less Wrong Discussion
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It's highly unlikely a priori that any being would have your exact experiences, but the higher the proportion of human-level life-forms, the more likely it is for there to be one. As such, knowledge that there is one is evidence of a higher proportion of human-level life-forms.
That's what I originally thought, but the problem is that the probabilities of each life-form having your experiences are not independent. Once we know that one (non-simulated) life-form has your experiences in our region of the universe, this precludes other life-forms having those exact experiences, because the other life forms exist somewhere else, on different-looking planets, and so can't observe exactly what you do.
Given our set of experiences, we filter down the set of possible hypotheses to those that are consistent with our experiences. Of the (non-simulation) hypotheses that remain, they all contain only one copy of us in our local region of the universe.
They don't necessarily exist on different-looking planets. It's highly unlikely that two planets will look exactly the same, but that's just because it's so unlikely for a planet to look exactly like that to begin with. It's not that one planet looking like that prevents another planet from doing so.
A given hypothesis with many human-level lifeforms is less likely to be filtered out than one with few. For example, imagine that it's just as likely a priori for there to be one set as two. There's a 25% chance of life on Earth, a 25% chance of life on Alpha Centari, and a 50% chance of life on both. Then we filter out all the ones without life on Earth, and we're stuck with a 33% chance of life on Earth and a 67% chance of life on both.
Thanks! What you explain in your second paragraph was what I was missing. The distinction isn't between hypotheses where there's one copy of me versus several (those don't work) but rather between hypotheses where there's one copy of me versus none, and an early filter falsely predicts lots of "none"s.