Recently, the Washington Post published a page one story headlined (in the print edition): "Proof of Life in Space Still Stubbornly Elusive." Here is the subtitle:

Mars has yet to deliver despite scientists’ extraterrestrial hopes.

The news was that NASA’s Curiosity rover had failed to detect methane in the Martian atmosphere, methane being a frequent byproduct of living organisms. "The new find wasn’t a total showstopper," the journalist Joel Achenbach added, "but scientists would have been thrilled by a different result." Why thrilled? Yes, they would have been thrilled by mere life. But what they are really searching for is extraterrestrial intelligence (SETI). Why?

Part of the answer is simple curiosity, of course. H.G. Wells and others entertained us with stories of extraterrestrial life and invasions. The vast genre of science fiction has often been based on the belief that others are out there — some of them with their telescopes trained on us. For a few decades in the 19th century scientists believed there were artificial canals on Mars. They were even sketched for our benefit.

Here are a few book titles, all published in the 1990s: We Are Not Alone (1993); Is Anyone Out There? (1992), Extraterrestrials: Where Are They? (1995), A Brief History of Life on Other Worlds (1998), The Hunt for Life on Mars (1997), After Contact: The Human Response to Extraterrestrial Life (1997). Since 1981, four books have been published with the title Are We Alone?

The late novelist Michael Crichton turned this into comedy in a lecture at Caltech in 2003 — "Aliens Cause Global Warming." There is "not a single shred of evidence for any other life forms and in forty years none has been discovered. SETI is a religion," he said. Make that fifty years now.

Crichton added a brief tour of nuclear winter, second-hand smoke and global warming, wherein science always defers to politics. We are seeing a "loosening of the definition of what constitutes legitimate scientific procedure," he concluded.

But he avoided what may be the most interesting question: Why have we invested so much hope in the search for extraterrestrial intelligence? Walter Sullivan, the late science editor of the New York Times, was a sober reporter. So why was his book titled We Are Not Alone, when there was no evidence for that claim?

Carl Sagan, the most widely publicized SETI-promoter of recent times, was once absorbed by flying-saucer reports. He joined the astronomy department at Cornell, chaired respectable conferences, and appeared on the Johnny Carson Show forty times. His biographer said that he "believed in superior beings in space, creatures so intelligent, so powerful as to resemble gods."

At the first international SETI conference, in 1971, Sagan declared that a new civilization is formed in the Milky Way every ten years: "There are a million technical civilizations in the Galaxy," he said. His biographer said that Sagan "believed in superior civilizations because he believed in Progress." We are reminded that at the end of The Origin of Species, Darwin foresaw a future in which "all corporeal and mental endowments will tend to progress toward perfection."

Built into this is the assumption that self-replicating life can easily get started on its own. No divine intervention is allowed, of course. That has been the underlying assumption since Darwin’s day. If so, life must have arisen by accident. Then it keeps climbing onward and upward, through random variation and natural selection.

In 1950, at Los Alamos, the nuclear physicists Enrico Fermi, Edward Teller and others were discussing the latest ideas about the universe. It was much larger and older than once thought; billions of stars evolving over billions of years, surely accompanied by more planets than stars. At that point the difficulty of life starting up on its own was considered to be exaggerated. If conditions are suitable and given enough time, life and evolution will happen.

Fermi then added his caveat. "So where is everybody?" It became known as the Fermi Paradox. SETI soon went into high gear.

Frank Drake, a Cornell astronomer, took it upon himself to demonstrate that life arising by chance was highly probable. By 1959, scientists could listen for signals from aliens using radio telescopes and a listening post was set up in West Virginia. Drake cobbled together what became known as the Drake Equation. It estimates the probability that intelligent beings are out there somewhere.

You take the number of stars and multiply by the fraction that have planets, times the number of planets per star, times the fraction within a habitable zone, times the likelihood of life evolving, times the probability of it reaching a level where critters can build radio transmitters — and so on.

But as Michael Crichton pointed out there was no data to work with. So enthusiasts were free to plug in their own numbers. That’s how Sagan came up with his "one million" civilizations. "Physics and chemistry are so constructed as to make the origin of life easy," he assured us.

But he was whistling in the dark. If the origin of life is easy, why can’t we make it happen in our own laboratories, by deliberate and ceaseless effort? Furthermore, Frank Drake and colleagues have been listening for over fifty years now but they haven’t heard anything much beyond background hiss.

There is another consideration. Some scientists want to believe in extraterrestrials because it is an article of our secular faith that there is nothing exceptional about human life. The Earth, its life, mankind and civilization are routine things; nothing out of the ordinary. If so, of course, we should expect to find such life all over the Galaxy.

Modern philosophers often want to rub our noses in the dust. Thou art dust! Some take pleasure in denigrating the human race, scorning us for promoting ourselves in the cosmic scheme. A little lower than the angels, indeed! Hadn’t we heard that Science had dethroned us long ago?

The sometimes misanthropic essayist Stephen Jay Gould derided our "need to see ourselves as separate and superior," and drew comfort from our downgraded status. Another extraterrestrial seeker said that finding life out there would deal another blow to our "psyche" — a blow that he thought was much needed.

The anti-religious physicist Lawrence Krauss wrote that the discovery of extraterrestrial life "would be far more jolting — and not just to orthodox Christians — than was the revelation that the Earth is not the center of the solar system."

Actually, the Copernican system wasn’t jolting at all. That is a modern invention, promoted by people like Gould and Krauss. The center of the cosmos was considered an insalubrious place, the point to which impure matter fell. It was "the physical correlate of humanity’s fallen state," wrote John Hedley Brooke, a professor of religion and science at Oxford. "To be placed on a planet was to move up-market."

The longshoreman philosopher Eric Hoffer — he worked on the San Francisco docks for 25 years — noted that intellectuals in the past century had done all in their power "to denude the human entity of its uniqueness"; to demonstrate that we are "not essentially distinct from other forms of life." By contrast, he noted, the 17th-century mathematician Blaise Pascal had said that "the firmament, the stars, the earth are not equal in value to the lowest human being."

Pascal’s polar opposite was the "humanist" Bertrand Russell, for whom "the stars, the wind in waste places mean more to me than even the human beings I love best." Today, we are inclined to prefer Russell to Pascal.

We have spent so much time putting mankind in the basement that it takes an effort to consider that we might be exceptional. Yet if we are alone in the cosmos — well, you can’t get more exceptional than that.

The search for extraterrestrial intelligence started at about the same time as the search for artificial intelligence here on Earth, beginning in 1956. Both varieties of intelligence have proved to be elusive — much harder to locate by radio or recreate in computers than anyone had imagined.

If we can’t find life on Mars, we are unlikely to find it anywhere else in our solar system. Maybe life takes a designer? You can see why SETI makes some of us anxious.