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dxu comments on Rationality Quotes November 2014 - Less Wrong
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I don't think the quote is talking about "hypothesizing" anything; I read it more as "You have to update on evidence whether that evidence fits into your original model of the world or not". Instead of "hypothesizing time travel when things don't make sense", it'd be more like a stranger appears in front of you in a flash of light with futuristic-looking technology, proves that he is genetically human, and claims to be from the future. In that case it doesn't matter what your priors were for something like that happening; it already happened, and crying "Impossible!" is as illegal a move in Bayes as moving your king into check is in chess.
Not that such a thing is likely to happen, of course, but if it did happen, would you sit back and claim it didn't because it "doesn't make sense"?
Yes. And then I would go see a psychologist. Because I find it more likely that I'm losing my grip on my own sanity than that I've just witnessed time travel.
Alright, so you bring this alleged time traveler with you to visit two or three different psychologists, all of whom are appropriately surprised by the whole 'time travel' thing but agree that you seem to be perceiving and processing the facts of the situation accurately.
Furthermore you have a lot of expensive tests run on the health and functionality of your brain, and all of the results turn out within normal limits. Camera-phone videos of the initial arrival are posted to the internet and after millions of views nobody can credibly figure out how it could have been faked. To the extent that introspection provides any meaningful data, you feel fine. In short, by every available test, your sanity is either far beyond retrieval down an indistinguishably perfect fantasy hole, or completely unmarred apart from perhaps a circumstantially-normal level of existential anxiety.
Now what?
Then I accept that there's a time traveler. The evidence in this second situation is quite a bit stronger than a personal observation, and would probably be enough to convince me.
Well, the insanity defense is always a possibility, but then again, you have no proof that you're not insane right now, either, so it seems to be a fully general counterargument that can apply at any time to any situation. Ignoring the possibility of insanity, would you see any point in refusing to update, i.e. claiming that what you just saw didn't happen?
It's always a possibility that I'm insane, but normally a fairly unlikely one.
The baseline hypothesis is (say) p = 0.999 that I'm sane, p = 0.0001 that I'm hallucinating. Let's further assume that if I'm hallucinating, there's a 2% chance that hallucination is about time travel. My prior is something like p = 0.000001 that time travel exists. If I assume those are the only two explanations of seeing a time traveler, (i.e. we're ignoring pranks and similar), my estimate of the probability that time travel exists would shift up to about 2% instead of 0.0001% -- a huge increase. The smart money (98%) is still on me hallucinating though.
If you screen out the insanity possibility, and any other possibility that gives better than 1 in a million chances of me seeing what appears to be a time traveler with what appears to be futuristic technology, yes, the time traveler hypothesis would dominate. However, the prior for that is quite low. There's a difference between "refusing to update" and "not updating far enough that one explanation is favored".
If I was abducted by aliens, my first inclination would likewise be to assume that I'm going insane -- this is despite the fact that nothing in the laws of physics precludes the existence of aliens. Are you saying that the average person who thinks they are abducted by aliens should trust their senses on that matter?
Ah. In that case, I think we're basically in agreement. To clarify: I only used the time travel as an example because that was the example that VAuroch used in his/her comment. I agree that even taking into account your observation of time travel, the posterior probability for your insanity is still much larger than the posterior probability for genuine time travel. You do agree, however, that even if you conclude that you are likely insane, the probability of time travel was still updated in a positive direction, right? It seems to me that Nominull (the person to whom I was originally replying) was implying that your probability estimate shouldn't change at all, because that's "clearly impossible"/"fictional evidence" or something along those lines. It is that implication which I disagree with; as long as you're not endorsing that implication, we're in agreement. (If Nominull is reading this and feels that I am mistaken in my reading of his/her comment, then he/she should feel free to clarify his/her meaning.)
Factually speaking, I think if you saw that happen, you would believe, regardless of your protestations now.
I don't think it's literally factually :-D
I think you're right. It's closer to, say... "serious counterfactually speaking".
Realistically speaking?
Unfortunately this still suffers from the whole "Time Traveller visits you" part of the claim - our language doesn't handle it well. It's a realistic claim about counterfactual response of a real brain to unrealistic stimulus.
I'll be sure to ask you the next time I need to write an imaginary comment.
It's not like anyone didn't know what I meant. What do you think of the actual content? How much do you trust faul_sname's claim that they wouldn't trust their own senses on a time-travel-like improbability?
Anecdotal evidence of the reaction of normal people to seeing something impossible:
http://www.bcgreen.com/comments/not_kansas.html
I wasn't the pedant. I was the tangential-pedantry analyzer. Ask Lumifer.
Your comment was fine. It would be true of most people, I'm not sure if Faul is one of the exceptions.