You could say I lost my gullibility at the movies. It was at in a movie theatre some years ago I was first exposed to the most useful guideline for dealing with extraordinary ideas, whether they be paranormal, scientific, religious or political.

It’s a centuries-old rule of thumb for scientists, philosophers and journalists, among others. But to explain how I hit upon it, I have to make a confession that’s embarrassing for a skeptic.

I once believed in ancient astronauts—the idea that Earth had long ago been visited by aliens and that many of the great achievements of old civilizations, like the Egyptian pyramids and Mayan art, were proof of their stay here. There was quite a craze for this theory in the 1970s with the publication of Erich von Daniken’s best-selling Chariots of the Gods. That book and others like it are still being read and believed.

For me though, the craze died in 1973 in a second-run movie house.

I was watching a documentary based on von Daniken’s books. On the screen were images of foreign-looking statues on Easter Island and animal outlines drawn on South American plains from apparent aerial perspectives. Ancient murals depicted, as the film would have it, the ETs themselves, piloting their spacecrafts. Proofs of alien presence.

And then appeared a small mysterious stone object. The film narrator suggested this could be an ancient replica of an extraterrestrial’s device, or perhaps a remnant of their spaceship.

“It’s a wrench!” called out a voice in the audience. And the theatre erupted with laughter.

The supposedly mysterious object did indeed look like a common, earthly tool, once you stopped trying to see it as a device from Betelgeuse.

After that, I viewed the rest of the film more skeptically, noting that elaborate stories were being concocted at every turn to fit everything into the gods-from-space hypothesis, when simpler and more likely explanations were always available.

This was my practical introduction to the principle of parsimony. The principle is more commonly known as Ockham’s Razor and it states the simplest explanation that accounts for all the facts is usually the right one. Even more commonly, it’s the KISS principle: Keep It Simple, Stupid.

You already use Ockham’s Razor, whether you know it or not.

A friend is half an hour late to meet you after work. What do you do? You could call the police to report your friend missing. Or call the hospitals to see if there’s been an awful accident. Or run through the streets alerting the populace that aliens have abducted another victim.

But what you usually do is figure your friend is held up in traffic and you wait a little longer. All the explanations for your friend’s delay are at least logically possible. But you settle on the simplest explanation that fits the facts, one that doesn’t require making up esoteric scenarios.

Scientists think the same way. Faced with various hypotheses about how the earth was formed or how the pyramids were built or what killed the dinosaurs, they wield Ockham’s Razor to select the theories that cover all the facts without bringing in additional, unneeded premises or unnecessarily complicated scenarios.

Ockham’s Razor, often attributed to 14th-century philosopher William Ockham (also spelled Occam), warned that one should not multiply entities needlessly. This applies very neatly to stories of ancient astronauts, ghosts, UFOs, Sasquatch, devils and other denizens of the paranormal world. In most cases, if not all, we don’t need to postulate such incredible entities to explain the facts. We have explanations that do no not require us to multiply entities.

Does this mean we have proven these entities don’t exist? No. Ockham’s Razor does not give us proof. Too often I’ve seen skeptics act as if it does: something can be explained simply and therefore the simpler explanation (fraud, a hoax, insanity on the part of witnesses, writing off all UFO reports as marsh gas or Venus sightings without investigating the actual circumstances, etc.) is taken to be the one and only explanation. Rather Ockham and the law of parsimony give us only the most promising explanation, showing us the route most likely to lead to the actual answer — and only if that explanation covers all the known facts.

But Ockham and the law of parsimony do cast serious doubt upon the existence of such paranormal entities. It is of course remotely possible, for example, that extraterrestrials evolved and developed advanced technology on another planet, travelled light years in space to come to this planet, decided to help earthlings build monuments for purposes that remain enigmatic, departed without leaving any clear messages, and haven’t been back to work openly with us ever since. It is remotely possible. But it is much more likely that ancient peoples built their own monuments using the down-to-earth skills we know they had.

There is no reason to jump to esoteric explanations when ordinary ones already account for all the evidence.

Unless you’re writing a bestseller or making a movie.