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Comment by delichon | Hacker News Reader

In 1979 I went to freshman orientation at college and attended a welcome speech from Carl Sagan. He asked us to shout out our estimate of the odds that UFOs are visitors from other planets. As a skeptic I was thinking of a low number. My smarter friend in the next seat whispered his answer to me, "fifty fifty".

Then Sagan said his estimate was fifty fifty. He told us about the principle of indifference and said that was the correct prior for a binary question when you have no data.

Since I have low confidence in the available data my own p(doom) is 50%. I'm looking for evidence to bump it either way.

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The Lindy effect [1] and the doomsday argument [2] are sort of corollary to your use of the principle of indifference.

p(doom) may be 50%, but the future life expectancy of humanity is at least several thousand years with 95% confidence.

[1]: https://en.wikipedia.org/wiki/Lindy_effect

[2]: https://en.wikipedia.org/wiki/Doomsday_argument

That was a very dumb thing for Sagan to say. It is very rare for anyone to be interested in any question about which he has absolutely no evidence. The reason that flipping coins is a thing is that humans intentionally searched for ways to artificially create the very rare situation in which a person genuinely does not have any evidence that would favor one outcome over another. (Human performed this search because humans prefer to play games where both participants have a decent chance of winning).

The complete lack of any signs of any alien civilization capable of making UFOs in the tremendous volume of astronomical, geological and geographical observations is evidence that tilts the probability far away from 50-50.

On one level sure. On another level - either UFO's are aliens or they're not. 50/50

You don't get to switch back and forth between levels. That's an error just like 2 + 2 = 5 is an error.

The only way you arrive at 50-50 without having made an error is if you give the matter only a few seconds of thought before moving on to other subjects, but that is clearly not the mental state Sagan was in: he had testified before Congress about this very matter.

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