Weird ideas can flourish if enough people share a false preconception, and here’s a marvelous article on the history and philosophy of widely held certainty that other planets were inhabited by people. Not just any people, either: good Christian people.

By the 1700s, there could no longer be any doubt. Earth was just one of many worlds orbiting the Sun, which forced scientists and theologians alike to ponder a tricky question. Would God really have bothered to create empty worlds?

To many thinkers, the answer was an emphatic “no,” and so cosmic pluralism – the idea that every world is inhabited, often including the Sun – was born. And this was no fringe theory. Many of the preeminent astronomers of the 18th and 19th century, including Uranus discoverer Sir William Herschel, believed in it wholeheartedly, as did other legendary thinkers like John Locke and Benjamin Franklin. How could so many geniuses believe in something so silly?

It’s a good read. The key idea that was leading everyone to this patently false conclusion was teleology, the notion that everything in the universe had a purpose, coupled to another belief, that that purpose had to be us.

Lest you think this is just ancient history and that we’ve moved beyond it, here’s a story about a contemporary crank with peculiar ideas about alien life.

Speaking at an international forum dedicated to the search for extraterrestrial life, Finkelstein said 10 percent of the known planets circling suns in the galaxy resemble Earth.

If water can be found there, then so can life, he said, adding that aliens would most likely resemble humans with two arms, two legs and a head.

“They may have different color skin, but even we have that,” he said.

Andrei Finkelstein runs a program that resembles SETI — and if I wanted to start a real argument here I’d tell you that SETI is about as quaintly absurd as Herschel’s belief that people lived on the moon. So I won’t tell you that. Yet.