Mac Tonnies, at his always thought-provoking blog Posthuman Blues, has been speculating lately about the relative likelihood of the two primary non-mundane explanations for the UFO phenomenon, the Extra-terrestrial Hypothesis (ETH), i.e. "they" are from other worlds, and the Extra (or Ultra) dimensional Hypothesis (EDH), i.e. that "they" are from some sort of other dimension, or parallel universe. See Mac's columns here and here for his recent columns on the subject, which seem to favour the EDH.
If the proverbial gun was put to my head, and I had to pick one of the two explanations, I go the other way. Call me a "nuts and bolts" guy, but if "they" are here, then I think it is most likely that "they" are coming "from away" as we say in the Maritimes - "away" being somewhere else in our galaxy, probably our local cosmic neighbourhood (to borrow one of Stan Friedman's favourite phrases).
Well, let me state first that I don't rule out the existence of other dimensions, or parallel universes, although I certainly don't accept them as some sort of proven reality. However, unless beings are travelling to our Earth from another Earth in some other dimension (the Sliders / Quantum Leap concept), or some other time perhaps (hello, Dr. Who), then they are by definition extraterrestrial, and so the ETH, even if it relies on the EDH or TDH for them to get here, is the most valid theory.
For me, the key thing is, the ETH is the one theory - other than the "its all explainable by terrestrial phenomena" theory - that we could actually accomplish, with our current level of knowledge (or something very close to it), should we ever decide to apply the resources necessary. If we accept that there is other life in our nearby cosmic neighborhood, which seems more and more likely with each passing year, then it is reasonably likely (at least 50/50) that they have that same technology, or better. That technology doesn't have to be thousands of years beyond our own - it may only have to be a few decades beyond our own. After all, look how far we've come in the past 100 years. Heck, look how far we've advanced in the past 10! Now, consider what we will be able to accomplish in 1,000 years, or say 5,000 - a blip on the universal clock. Michio Kaku has some interesting thoughts on this topic well worth reading.
To me, the people who seem to gravitate to the EDH are motivated by an inability to comprehend why an ET race would behave in the manner that some seem to, according to reports. Assuming those reports are true (a big assumption, but...), all I can say is "so, what makes it unlikely that they are ET?" After all, we know nothing about how an ET civilization would be structured - their social order, their behaviour, their concept of morality (if they even have one), their beliefs, etc. To assume that they would behave anything like us is the height of cultural hubris. To assume that they're coming here as a result of us, as opposed to some other reason, is also big leap. As far as "they" are concerned, we might occupy the same place on the evolutionary chain as an ant does to me when I walk down the sidewalk. I usually don't even notice them. Finally, to assume that "they" are biological entities is a big assumption as well. Odds are that "they" may be artificial life forms of some sort (here's hoping it's not an advance scouting party for the Borg, but something more along the lines of Tickle-Me-Elmo, or Jessica Simpson, pictured left, two of the more friendly artificial "life" forms on our planet).
Rather than grapple with these questions, however, the EDH supporters have basically thrown up their hands and said something to the effect of "well, aliens would never behave like that, so it must be some sort of extra / ultra-dimensional reality". Consider me unimpressed with that line of "reasoning".
It gets worse when people trot out the concept of "the Trickster" - as far as I'm concerned, you might as well be talking about leprechauns, or saying that it's all the work of Satan, because at this point you've moved beyond any real scientific reasoning, and into the land of myth, or belief. Nothing wrong with that, but just don't expect me to buy into it as a reasonable working hypothesis based on science, because it isn't. It's more suited for the Dungeons and Dragons set.
Anyway, as nobody knows for sure what UFOs are, much less where they might be from, then the debate is not much more than an interesting way to pass the time. The problem comes when one group insists, as they often do, that their "theory" is "fact". In my experience, this is usually the ETH die-hards (although I've noticed the EDH die-hards getting more vocal lately about the primacy of their view), which is where I part ways with them. I agree with them that their theory is more plausible than the EDH. But neither is anywhere near proven as a fact.
Still, when push comes to shove, I'm an ETH guy.
Emphasis always on the "H".