At a demonstration in Boston in February — a kind of dress rehearsal for the national March for Science planned for April 22 — some protesters carried signs that read like punch lines on the TV comedy “The Big Bang Theory.”

“π Is All the Irrationality I Need.” “If You’re Not Part of the Solution, You’re Part of the Precipitate.” “Alternative Facts are √−1.” If you didn’t know that the square root of negative one is what mathematicians call an imaginary number, you probably belonged at a different rally.

Other signs were more direct. “Stand Up for Science.” ”Follow the Evidence.” “Scientists Speaking Truth to Power.” But perhaps the most trenchant simply read “Objective Reality Exists.”

Stripped of its context, the slogan might have come from a cultural conservative attacking the postmodern left and the idea that all truth is relative — texts commenting on texts. The target of the protesters, of course, is the alternative reality of President Trump, who has called for major cuts to government agencies in charge of scientific research and policy: a 31 percent reduction in the budget of the Environmental Protection Agency, for example, and an 18 percent cut to the National Institutes of Health.

The president’s staunchest supporters believe, after all, that climate change is an illusion spun by environmentalists to subvert capitalism. The safety of vaccines, they say, is disinformation from government labs and the pharmaceutical industry.

But substitute “GMOs” for “vaccines,” and the argument could have come from Greenpeace. Suspicion of science — and the dismissal of scientific consensus as a conspiracy — has been building at both extremes, not just among conservatives. In a half twist, the political spectrum has looped back on itself to become a Möbius strip, where left and right blur together.

In a recent issue of The Chronicle of Higher Education, Alan Jay Levinovitz, who teaches a class on religious belief and medicine at James Madison University, described how his students, rising to the ideals of multiculturalism, responded uneasily when asked whether human health was really governed by four humors (as Aristotle taught), by seven chakras (nodes of spiritual energy in Hindu and Buddhist belief), or by African spirits.

The questions make them uncomfortable “and they soften their answer,” he wrote. For the students, it’s not true that there are four humors, but for other cultures it is.

Alternative facts. The position, Levinovitz writes, is “perilously similar” to what is heard from the White House. The subjective trumps the objective, and the postmodern ideas of academia may shoulder some of the blame. Science and shamanism are “different ways of knowing,” and “Reality,” with a capital R, is just one more ideology imposed by a privileged elite for its own economic benefit — an idea that might have come from Marx.

“We point out that one culture’s science is not another’s,” Levinovitz wrote, “that elites wrongly force standards of truth on the less powerful — and then expect people to trust a culture of elites telling us that vaccines are safe, that man-made global warming is real.”

Mainstream journalism has caught a touch of the fever. Political reporters have become so accustomed to writing about competing narratives that the implication seems to be that there is no bedrock, just stories all the way down.

While commentators in recent years have focused on the dangers they see posed by trigger warnings, safe spaces, microaggressions and other paraphernalia of the academic left, the relativism of the right has quietly thrived on the confusion, and may threaten to subsume us all.

Almost 40 years after William Jennings Bryan took on Clarence Darrow in the Scopes trial, Young Earth Creationists began taking a postmodern tack. Confronted with the stunning explanatory power of the theory of evolution, they devised what they saw as a competing model rooted in a fundamentalist interpretation of the Bible.

The believers didn’t see this as anti-science but as alt-science, including a rejiggering of radioisotope dating that, lo and behold, showed an Earth and a universe that was only about 10,000 years old.

They created a competing narrative. Evolution was just a credo of a different religion — naturalism or scientific materialism — which the biblical creationists cast as the invention of an oppressive elite. By the early 1990s, creationism had transformed into something slicker, the intelligent design movement.

At the other end of the political spectrum, meanwhile, Vine Deloria Jr., a Native American activist and professor, rejected mainstream geology and evolutionary biology in favor of an alternative science consistent with tribal creation beliefs, in which Indians may have walked the earth with dinosaurs.

By starting with different assumptions, you can construct a belief system that puts you and your compatriots at the center of a story you fervidly believe. Politics is the arena in which these fight to the death, or to some temporary compromise.

How do we adjudicate?

The power of science comes from the simplicity of its own credo, its underlying postulates: There is a universe that consists of what we can observe and measure. It operates according to principles that human brains can attempt to understand.

If some event appears to be caused by a supernatural agent, that just means we need more research. With enough scrutiny and study, the inexplicable will be roped into the natural realm.

That, anyway, is the ideal. Individual scientists have biases and make mistakes. Some even cheat. They do experiments that cannot be replicated. But in the long run these aberrations cancel out — plus added to minus is zero — and the search steadily converges on better approximations of the truth.

A consensus and not a conspiracy. Objective reality exists. These are ideas worth marching for, and they don’t all fit on placards.

Science doesn’t (or shouldn’t) profess to be The Truth. Or just another truth. It’s the best way humans, we blinkered souls, have found to arrive through observation and experiment at what seems for the moment to be true.

George Johnson is the author of “The Cancer Chronicles,” “The Ten Most Beautiful Experiments,” and seven other books, which have been translated into 15 languages. His column, “Raw Data,” appears in The New York Times. Twitter: @byGeorgeJohnson