We base that judgment on an empiric respect for evidence (reasoning about the origin of a rock starts with having a rock in front of you). The less (quantity) and the lower (quality) of the evidence presented, the more improbable the assertion.

Sure, many folks claim to have observed ufo’s, and then lots of them proceed to explain what they are and where they came from … contradicting the very definition “unidentified” flying objects. If they could suspend the urge to infer, and use the experience as a prompt to gather more data, they’d win a lot more acceptance, even from us skeptics.

When someone says he saw a ‘flying saucer’, we know only that went on inside the guy, and/or possibly in the world outside. When many folks say they saw the same thing, it increases the possibility that the ’something’ did occur in the outside world. But it still does not define what.

Even if we accept the observation as valid, we face many alternative explanations, both subjective and objective. They all need to be tested in the hard crucible of accord with fact — objects, events and processes.

One thing that always puzzles me is those scientists who dismiss reports of UFO abductions as nonsense, but don’t appear to find the fact that thousands of people firmly believe they have experienced them worthy of investigation. This is just one example of a certain attitude that says “it’s impossible, therefore we won’t look at what’s really happening”.