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%...
Another month, another rationality quotes thread. The rules are: