Every night after supper I gather the kids in the living room and we build a fire and make some popcorn and sit in a circle on the rug, and I announce, "Girls, it's time to discuss the implications of the Fermi Paradox."

"AGAIN?????" they say.

You have to pound the serious philosophical material into their little noggins before the hormones kick in and they discover Ricky Martin and the Backstreet Boys and suddenly start adorning their walls with giant diagrams of the human reproductive system. At least I hear that's what the teenage years are like.

The Fermi Paradox is named after Enrico Fermi, the nuclear physicist, who was having lunch with colleagues in Los Alamos, New Mexico, in 1950, and began discussing how human beings would eventually master ultra-fast space travel and potentially colonize the entire galaxy. And then he suddenly said something like: "Where is everybody?"

The other chaps at the table no doubt stopped chewing their tuna sandwiches and stared at Enrico and wondered if the radiation had finally zapped his brain. At some point they figured out what he was driving at: Why haven't older, more advanced alien civilizations already colonized the galaxy? Is it because . . . they don't exist?

In the two-hour PBS special "Life Beyond Earth," an admirable piece of work that airs tonight, Timothy Ferris, a truly great science writer and pretty decent television narrator when he's not trying to talk while eating sunflower seeds, tries to illustrate the flaw with the Fermi Paradox. He gets dressed up for an elegant lobster dinner in his home. He opens the front door and waits for a lobster to show up and crawl onto his plate. Hours pass. No lobster. Finally he concludes that he has failed to take the lobster's preferences into account. "Lobsters have their own agenda. They don't want to come to my house."

Clever, yes, but the viewer doesn't exactly feel SATED after the Ferris lobster experiment.

The Fermi Paradox does have some supporters out there. They say that even if we assume that most alien civilizations choose to stay home and contemplate their navels, or whatever anatomical feature they have on their bellies, it strains credulity to think that ALL alien civilizations will decline to colonize the galaxy. Therefore, say these folks, extraterrestrial intelligence must be extremely rare or non-existence.

The problem is, everyone on all sides of this issue is wrestling with vapor. There are no facts here. The three most honest words in the realm of science are "I don't know." It's when you stray from that core principle -- the acknowledgment, indeed the embrace, of one's ignorance -- that you get into trouble.

As Ferris points out, we don't yet know of any life beyond Earth and don't really know how life comes about. If life is the natural result of ordinary chemical interactions, then it's probably all over the universe. But if it's an "extraordinary fluke," as one scientist puts it, then we won't find much life out there. Both extremes are still in play.

The next huge uncertainty is whether intelligence commonly emerges on worlds with living things. You can argue it either way, based on our own example of life on Earth. Yes, we can declare that we do have intelligent life here, notwithstanding the various recent actions of Donald Trump. But there have also been about a billion other species that weren't intelligent. For three billion years or so, all life on Earth was microbial. And those microbes were happy! They had NO DESIRE to turn into human beings.

Here is a description, from the book "A Walk Through Time," of the most exciting events on the Earth about 2.7 billion years ago:

"Special joint ventures occur in communities of mixed populations. A sluggish, ancient fermenting bacterium and a small, swimming, spirochete-like bacterium may have formed a particularly brilliant partnership . . . "

That was as good as it got. That was Saturday night in the big city. One piece of scum gradual oozes and ferments its way into the heart of another piece of scum. Eventually you wind up, after another billion years or so, with the dramatic emergence of an actual ameoba.

But let's go on and stipulate that, somehow, life gives rise to intelligent creatures. Now we get to the assumption that Ferris calls into question, which is that these creatures will build spaceships and seek to colonize the galaxy. But it's not clear that star trekking makes any sense at all. We turn ourselves into an invasive species, carrying our germs and our cocky attitude to places where human beings are not indigenous. Worse, we invite alien organisms to come back to our home planet and potentially wreak havoc. (Why do things only wreak "havoc"? Why can't you wreak a mild disturbance?)

Humans may, in fact, be turning inward. The freakiest science fiction scenarios today involve the convergence of mind and machine, the downloading of consciousness into silicon networks. Personally I haven't left my computer terminal since August.

So Ferris is right, the Fermi Paradox shouldn't be viewed as "evidence" that there aren't intelligent civilizations. But at the same time, there's no evidence for such civilizations, either. If anything, our tendency in recent decades has been toward excessive optimism. We grant to the aliens all of our own evolutionary adaptations and much, much more. We always assume that they have incredibly nifty spaceships that go faster than ours. And they never have a problem with the Bubble Boy Virus.

The only way to know the truth on the alien question is to get out there and find some actual data. Fermi's lunchtime question, and Ferris's lobster dinner, are interesting thought experiments, but that's about all. A good rule is, don't try to solve the secrets of the universe and eat at the same time.

(Rough Draft oozes and ferments its way onto the washingtonpost.com Web site at 1 p.m. on Monday, Wednesday and Friday.)