In the spirit of posting numbered sets of principles in this forum, here's some more. Actually, they aren't exactly proposed rules, they are different ways that people seem to interpret the term 'UFO', along with my comments, criticism and suggestions. 1. A 'UFO' is indeed unidentified, but only because of a lack of information about it. If sufficient information was available, then it would resolve into a meteorological, astronomical or optical phenomenon of a known sort. This is the belief, often left unstated, that seemingly defines the "skeptic". This belief does have a grain of truth to it, since it kind of describes the research-program to which sightings are subjected in hopes of identifying them. But when applied as a general principle to all UFO sightings, the a-priori assumption that all of them could be shown to have mundane explanations if enough were known about them looks to me like an insupportable article of faith that goes well beyond the (lack of) evidence. 2. A 'UFO' is exactly that, an 'unidentified flying object'. As long as it's unidentified, it's unidentified, so by definition we don't know what it is. That doesn't necessarily mean 'unidentifiable' or imply that we couldn't discover what it is if more information was available, but with the information that we have, we can't reach a conclusion. Hence drawing premature conclusions about what it is, is probably a mistake. That obviously applies to "They are aliens!" But it applies equally well to "UFOs are bullshit!" This is the position that I lean towards: If it hasn't been identified, then don't get out in front of the (lack of) data by prejudging what it is or what it has to be. Seek more information if it's available. It's fine to form hypotheses, but they should serve as guides as to what kind of additional information we would like to acquire. Speculative hypotheses shouldn't slide over the line into being unsubstantiated conclusions. 3. A 'UFO' is an 'unidentifiable flying object' (as opposed to 'unidentified'). If interpreted literally, this idea would indicate a sort of UFO-mysterianism, the idea that whatever UFO's are, they transcend human cognition. (We sometimes see similar ideas proposed regarding religious experience, which UFO experience so resembles.) I'm not sure what would justify a belief like this regarding UFO's. (There are theological justifications for the religious version, such as maintaining divine transcendence.) 4. A 'UFO' is an unidentifiable flying object, but not unidentifiable in principle as in #3 above, but only unidentifiable in terms of current assumptions. It's basically the belief that UFO's, or at least the most difficult residual cases that can't easily and plausibly be reduced to mundane phenomena, must have extra-mundane explanations. Like #1 it seems to me to get too far out in front of the data. 5. 'UFO' means 'alien spaceship'. Lots of "believers" seem locked into this interpretation. This one seems to obviously fall prey to the 'premature conclusion' and 'unjustified assumption' objections. 6. 'UFO's' (or at least the more difficult residual cases) might be all manner of things: space-aliens, time-travelers, interdimensional- visitors, space animals sounding in Earth's atmosphere, as-yet unknown physical phenomena or an unknown (and unknowable) number of unsuspected possibilities. The science-fiction nut in me finds a grain of truth in this one since it suggests keeping an open mind. (If some 'UFO' reports do have an extra-mundane explanation, it might turn out to be something we don't even currently imagine.) But once again, confusing speculations for conclusions is unjustifiable and way out in front of the evidence.