"I don't think that generic aliens should be considered especially improbable a priori - before the evidence is considered. I think that they are unlikely a posteriori - based on the fact that we don't see them"
Citation?
There's plenty of evidence for non-man made, non-hoaxed, non-astronomical, non-weatherrealated unidentified flying objects according to studies made by the US and French military:
http://en.wikipedia.org/wiki/Project_Blue_Book#Project_Blue_Book_Special_Report_No._14
most important highlights:
http://lesswrong.com/lw/ffd/struck_with_a_belief_in_alien_presence/7t4i
The black swan example was just a general pondering.
"I don't think that generic aliens should be considered especially improbable a priori - before the evidence is considered. I think that they are unlikely a posteriori - based on the fact that we don't see them. I think that any intelligent space-faring life would be busy building spheres around stars (if not outright disassembling the stars) as quickly as they spread out into the cosmos. So we'd notice them by the wake of solar systems going dark. At the very least, there's no reason to think that they would hide from us, which is what these scenarios tend to require"
This is very speculative to me. I don't think we can use it as evidence for or against.
Even if you could rule out man-made and weather-related causes for some UFOs, that wouldn't imply that they were caused by an extra-terrestrial civilization either. Some UFOs may still be unexplained, but all that means is that we don't know enough about them to say what they are.
That said, I don't think you can rule out weather and human craft. Others have already explained why I find the "primary" evidence unconvincing.
This is very speculative to me. I don't think we can use it as evidence for or against.
Let me put it this way. My guess ...
Recently I've been struck with a belief in Aliens being present on this Earth. It happened after I watched this documenary (and subsequently several others). My feeling of belief is not particular interesting in itself - I could be lunatic or otherwise psychological dysfunctional. What I'm interested in knowing is to what extend other people, who consider themselves rationalists, feel belief in the existence of aliens on this earth, after watching this documentary. Is anyone willing to try and watch it and then report back?
Another question arising in this matter is how to treat evidence of extraordinary things. Should one require 'extraordinary evidence for extraordinary claims'? I somehow feel that this notion is misguided - it discriminates evidence prior to observation. That is not the right time to start discriminating. At most we should ascribe a prior probability of zero and then do some Bayesian updating to get a posterior. Hmm, if no one has seen a black swan and some bayesian thinking person then sees a black swan a) in the distance or b) up front, what will his a posterior probability of the existence of black swans then be?