I went to sleep last night pondering the "men-in-black" phenomenon, and generally thinking that the overall UFO experience is unlikely to be material craft piloted by biological entities from other planets--for reasons apparent to many students of the same. Basic common sense (and a rudimentary understanding of science) is all you need to have. Even assuming that biological life is universal--a reasonable assumption--intelligent life would be rare. We have evidence that it's evolved only once in the billion-or-so years on Earth (although I suspect that it's happened more than once), and sapient, biological life would probably have a short life span--a few thousand years--before it self-destructs, or evolves out of the physical plane.
Let's assume, for the sake of argument, that an intelligent biological species--one that is "intelligent" enough to manipulate the material of the host planet sufficiently to leave the host--has a life span of ten thousand earth years, with maybe one thousand years, max, of space travel, before it evolves out of the physical, or gets blasted by the Romulans. Such a species would require several hundred million earth years, under ideal conditions, to evolve from unicellular life, to a developed, space-faring biological entity, only to flame out (or leave the physical system) at the end.
(These limitations, by the way, is probably why intelligent life did not evolve on Mars--there simply was not enough time for it to do so, during the relatively brief period during which Mars was habitable.)
Now, these are very simplistic assumptions, and they are quite possibly incorrect--we simply do not have any data about the evolution of life on any planet other than Earth--but they are reasonable ones.
So, within (say) a one-thousand-lightyear radius of Earth, let's assume that intelligent life has evolved ten times, somewhere, on a habitable planet. Or maybe twenty times. It doesn't really matter.
Let's say that such a species developed a warp drive and could beat the speed of light, so that they could even travel to Earth quickly if "they" wanted to. There is still one insurmountable barrier, and that barrier is time.... Not distance, as we might assume, but time. Assuming that an evolved biological species has a lifespan of several thousand years, their period of high technological development may have been several million years ago, or one hundred thousand years in the future. Such a species visiting Earth would have encountered either homo erectus preparing for an ice age, or the remnants of a nuclear holocaust or (more likely) a planet made less habitable by a runaway greenhouse effect. They would not encounter "us," "now."
It is still possible for such a species to visit us and harass UFO experiencers with non-disclosure threats, but only if they are proficient at time travel... And they would have to locate us, out of the vast expanse of the universe, not only "here," but "now."
Vast expanses of time as well as distance separate us, now, from any intelligent biological species that may have evolved elsewhere. The casual ET hypothesis believer has a mental picture of hundreds, maybe thousands, of intelligent biological species, all existing "now" and all visiting Earth, in our time, in our space, to conduct experiments that superficially resemble our scientific method. While it's possible, it's not likely. Any beings visiting us "now" are likely doing so in a form that is incomprehensible to us, and are only appearing as physical simply because we have no other way (yet) of perceiving them. They have evolved beyond our limited physical form.
Again, all of the above is based on very primitive assumptions based on a rudimentary understanding of physical science, and are probably "wrong"--but they are not unreasonable. They are more reasonable than the assumptions of those who argue that UFOs are physical craft built by biological entities, somewhat like us, from nearby planets.
These are my beliefs and assumptions, and I am constantly turning them over and examining them as I read the various (and intriguing) accounts of UFO experiencers, of which I am one.
So it was very strange this morning to have an elaborate, vivid dream informing me that there exists an intelligent physical species on a planet around Proxima Centauri--not Alpha Centauri, but the red dwarf Proxima Centauri, 4.2 lightyears away. With a warp drive, they could get here in a month. The dream flies not only in the face of everything I assume, but also of what I know.... We all know that the two primary alien races visiting Earth are the water-based evolved beings from the Pleiades, and the evil grays from Zeta Reticuli--NOT Proxima Centauri.
So, I await further contact, and will revise my assumptions accordingly.