What Makes a Claim "Extraordinary" in Bayesian Terms?
A claim warrants a low prior when it conflicts with, or requires significant additions to, well-established, high-confidence background knowledge. The prior isn't set by how
surprising something feels — that's a psychological criterion, not an epistemological one. It's set by how well the claim coheres with everything else we already have strong evidence for. So the burden is to show that H1 genuinely conflicts with high-confidence background knowledge, not merely that it feels strange.
The Strongest Arguments That H1 Earns a Low Prior
1. It requires violations of, or massive extensions to, well-tested physics.
Interstellar distances are not merely large — they are almost incomprehensibly hostile to travel. The nearest star system is ~4.2 light years away. Getting a meaningful payload there in a human lifetime requires either relativistic speeds — demanding energy outputs that dwarf anything in our engineering experience by many orders of magnitude — or exotic physics (wormholes, warp drives) that currently have no empirical support and, in some cases, appear to require negative energy densities that may be physically forbidden. This is a genuine, specific conflict with high-confidence physical knowledge. It doesn't make H1 impossible, but it assigns a concrete
engineering improbability to the hypothesis.
This is probably the strongest leg of the "extraordinary" argument.
2. The base rate of "anomalous phenomenon → genuinely novel ontological category" is very low.
History gives us a strong empirical prior here. Anomalous observations have repeatedly turned out to be prosaic phenomena misunderstood at the time: ball lightning, meteors, mirages, atmospheric optics, magnetic anomalies. The conversion rate of "unexplained" to "genuinely paradigm-breaking" is low, even when the unexplained cases seem compelling to observers. This is a legitimate base-rate argument, not just motivated skepticism.
3. H1 requires a conjunction of several independent improbabilities.
H1 as typically construed doesn't just require that non-human intelligence exists somewhere — it requires that it: exists, survived long enough to develop interstellar travel, decided to visit
this star system, is doing so
now (in the cosmically brief window of human observation), and is doing so in a way that produces the specific pattern of ambiguous, non-confirmable sightings we observe rather than something more unambiguous. Each step in that conjunction multiplies the improbability. This is a real and legitimate consideration.
Where the "Extraordinary Claim" Argument Weakens or Fails
Here is where your challenge bites hardest.
1. "Extraordinary" relative to what background knowledge?
The background knowledge cuts both ways. Yes, interstellar travel is hard by known physics. But the same background knowledge tells us the universe is 13.8 billion years old, contains hundreds of billions of galaxies each with hundreds of billions of stars, many with Earth-like planets, and that the chemistry of life appears to emerge readily under the right conditions. The prior probability that
no other intelligence has ever arisen and developed capabilities beyond ours in 13.8 billion years is itself not obviously high. You cannot selectively invoke background knowledge on only one side.
In other words: H1 conflicts with some background knowledge (physics of travel) while being
supported by other background knowledge (cosmic scale, age of universe, prevalence of habitable worlds). Calling it straightforwardly "extraordinary" smuggles in a selective reading of the background.
2. The claim can be formulated without requiring interstellar travel.
H1 as I stated it is:
"some UAPs are from an advanced, non-human intelligence we don't understand." That formulation doesn't specify extraterrestrial origin. It's logically compatible with something like an ancient terrestrial intelligence, a non-biological intelligence arising from processes we don't understand, or something genuinely outside our current ontological categories. If you don't require interstellar travel, the strongest physics-based argument for a low prior evaporates. I was essentially arguing against a more specific version of H1 than the one actually stated.
3. The label "extraordinary" can smuggle in psychological bias.
There's a documented phenomenon where the scientific community has systematically under-investigated UAPs for decades largely because of social and reputational costs — not because of careful Bayesian analysis. The 2023 NASA UAP report explicitly acknowledged this. When professional scientists avoid a topic because it's associated with fringe culture, and then we cite the
lack of scientific investigation as evidence for a low prior, we've created a self-sealing argument. The prior is partly a product of the same bias that suppressed investigation, not purely independent evidence.
