The term \”pseudoscience\” gets thrown around quite a bit these days, most notably in debates about the dominant consensus on anthropogenic climate change. Say \”pseudoscience,\” and immediately a bunch of doctrines leap to mind: astrology, phrenology, eugenics, ufology, and so on. Do they have anything in common? Some posit unknown forces of nature, others don\’t. Some are advocated by outsiders to the scientific community, while others have been backed by the elite. And the status of each can fluctuate over time. (Astrology, for example, was considered an exemplary field of natural knowledge from antiquity through the Renaissance.)

For millennia, philosophers have attempted to erect a boundary between those domains of knowledge that are legitimate and those that are anything but­—from Hippocrates\’ essay on \”the sacred disease\” (epilepsy) to editorials decrying creationism. The renowned philosopher Karl Popper coined the term \”demarcation problem\” to describe the quest to distinguish science from pseudoscience. He also proposed a solution. As Popper argued in a 1953 lecture, \”The criterion of the scientific status of a theory is its falsifiability.\” In other words, if a theory articulates which empirical conditions would invalidate it, then the theory is scientific; if it doesn\’t, it\’s pseudoscience.

That seems clear enough. Unfortunately, it doesn\’t work. Epistemologists present several challenges to Popper\’s argument. First, how would you know when a theory has been falsified? Suppose you are testing a particular claim using a mass spectrometer, and you get a disagreeing result. The theory might be falsified, or your mass spectrometer could be on the fritz. Scientists do not actually troll the literature with a falsifiability detector, knocking out erroneous claims right and left. Rather, they consider their instruments, other possible explanations, alternative data sets, and so on. Rendering a theory false is a lot more complicated than Popper imagined—and thus determining what is, in principle, falsifiable is fairly muddled.

The second problem is that Popper fails to demarcate in the right place. Creationism, for example, makes a series of falsifiable claims about radioactive dating, rates of erosion, and so on, while the more \”historical\” sciences, like geology and astronomy, pose theories that are more explanatory narratives than up-or-down (and therefore falsifiable) protocol statements of empirical bullet points. Any criterion had better at least replicate our common-sense notion of \”science,\” and so far no clear criterion has been able to do so. No wonder most philosophers have given up on the task. As the prominent philosopher of science Larry Laudan put it 30 years ago: \”If we would stand up and be counted on the side of reason, we ought to drop terms like \’pseudoscience\’ and \’unscientific\’ from our vocabulary; they are just hollow phrases that do only emotive work for us.\” Demarcation is distinctly out of fashion among philosophers today.

On the other hand, \”emotive work\” is pretty interesting from a historical perspective. Scientists consider a great many doctrines to be wrong, even wrongheaded, but not all of them get labeled \”pseudoscience.\” No one in the history of the world has ever considered himself a pseudoscientist. It is a term of abuse that is deployed by some members of a scientific community against individuals they consider threatening. By tracking under which conditions scientists denigrate others as \”pseudoscientists,\” we can actually learn how scientists define healthy science at a particular moment. Instead of attempting to find a one-size-fits-all demarcation criterion, we should think about pseudoscience historically. This helps us understand how science functioned in the past as well as in the present.

Over the past several years, I\’ve undertaken to do just that, in studying Immanuel Velikovsky. Velikovsky (1895-1979) is no longer a household name—very few people under 50 have heard of him—but from 1950 to 1980 he dominated debates about demarcation. At issue were his catastrophist theories, first promulgated in his 1950 blockbuster Worlds in Collision (published by Macmillan, then the most respected publisher of scientific books in the United States), and later extended and elaborated in a half-dozen further volumes.

Velikovsky had a big idea. When he read ancient myths and legends from around the world—especially the Hebrew Bible and other texts from the ancient Near East—he came across similar images: fire raining from the heavens, enormous earthquakes, epic flooding, and so on. What if these were not just metaphors or hallucinations, but actual eyewitness observations? What if they described not different disasters, but one single global catastrophe? Velikovsky claimed that by properly correlating and interpreting these texts, one could deduce the outlines of a series of celestial catastrophes, beginning around 1500 BC.

In brief, according to Velikovsky, a comet was ejected from Jupiter and became gravitationally and electromagnetically trapped by Earth, wreaking enormous trauma on our planet. After breaking free and struggling with a displaced Mars, the comet settled into an orbit around our Sun. We now call this destructive comet Venus. Velikovsky\’s theory unified an idiosyncratic version of ancient history with a new account of the solar system; it also contravened every accepted premise of geology, paleontology, and celestial mechanics.

The fate of Velikovsky\’s theory is instructive for two principal reasons. First, Worlds in Collision was, so to speak, \”born pseudoscientific.\” Before it, fringe doctrines (say, parapsychology or phrenology) had been introduced by a given scientist, a lively debate ensued, and those doctrines were then excised (and their proponents exiled) from the scientific community. Not so with Velikovsky. Although he was trained as a medical doctor and psychoanalyst, he was not a member of any of the communities with which his book engaged. His theories were not discussed dispassionately and then set aside; they were vehemently attacked even before the book appeared (the advance publicity set certain people off), the publisher was threatened with a boycott, and for decades he remained a prime target for self-appointed demarcators, including the distinguished astronomers Harlow Shapley and Carl Sagan.

The emergence of this new method of policing pseudoscience says a lot about the organization of science during the cold war. In the geopolitical clash between the United States and the Soviet Union, science and technology assumed a central place (think of nuclear weapons, or Sputnik). As a result, science was better financed, more visible, and more prestigious than ever, but also laden with newfound anxieties about oversight and integrity.

A second reason to focus on Velikovsky is the nature of the evidence. Most fringe doctrines do not survive their creators; with their deaths comes a cleaning of the attic and a trip to the dump. But in 2005, Firestone Library at Princeton University announced the opening of the Immanuel Velikovsky Papers to researchers. (Velikovsky had lived in Princeton, N.J., from 1952 until his death, and he was a frequent presence in the library and around town, although he never had any affiliation with the university.) I went to take a look, the name striking a chord from my youthful reading of UFO lore and other nerdy arcana.

His papers are among the most comprehensive personal archives I have ever seen, spanning 65 linear feet of material: drafts of manuscripts, fan mail, hate mail, assorted correspondence, and much more. Here we can trace the microdynamics of a demonized theory from birth, charting its rise in popularity and eventually its fairly sudden senescence after Velikovsky\’s death.

One could do a lot with this material: elucidate how both mainstream and marginal publishing worked, for example, or track how social movements on the fringe coalesce and develop. I chose to sift through Velikovsky\’s manuscripts and explore the issue of pseudoscience. A few hours poring over the vitriolic correspondence by scientists excoriating Macmillan for publishing Worlds in Collision and threatening a boycott of the press were so fascinating that I felt I needed to keep reading about how boundaries of science were being defended. I was most struck by one dominant theme running through all the pro- and anti-Velikovsky documents: Everyone demarcates.

After the publishing succès de scandale of 1950, Velikovsky stepped back from heated confrontation with what he and others came to term \”establishment science.\” Instead, he courted his fellow Princeton resident Albert Einstein for legitimacy and sought to bolster the scenario from Worlds in Collision with claims that discoveries from the emergent Space Age confirmed his theories about Venus and other planets. He tried to establish himself through testimonials from scientific authorities and validated predictions as a legitimate scientist, not a crank. When some creationists attempted to tie their theory of a global flood to Velikovsky\’s cosmic catastrophism, he counterattacked ferociously, arguing that their fusion of science and religion was distinctly unscientific. Likewise, John C. Whitcomb and Henry Morris, authors of The Genesis Flood—the book that resurrected flood geology as the basis for scientific creationism—purged hints of Velikovskianism from their text. He was too \”pseudo\” for the creationists, and vice versa.

There is an important lesson in this. All so-called pseudoscientists believe they are simply scientists, albeit ones with heterodox views marginalized by the mainstream. (They aren\’t necessarily right—many people have mistaken self-conceptions.) But to be a scientist, you need to behave like one, and one thing scientists do constantly is, well, demarcate. Velikovsky and his peers knew there was an edge to legitimate science, and they policed it very carefully, just like \”establishment\” scientists did and continue to do.

I have come to think of pseudoscience as science\’s shadow. A shadow is cast by something; it has no substance of its own. The same is true for these doctrines on the fringe. If scientists use some criterion such as peer review to demarcate, so will the fringe (creationists have peer-reviewed journals, as did Velikovskians). The brighter the light of science—that is, the greater its cultural prestige and authority—the sharper the shadow, and the more the fringe flourishes.

Fringe theories proliferate because the status of science is high and science is seen as something worth emulating. Since World War II, science has been consistently prestigious, and heterodox doctrines have proliferated, but the pattern holds in the past as well. Late Enlightenment France and Victorian Britain were high points of scientists\’ status, and clusters of such movements (mesmerism, spiritualism, phrenology) cropped up at these moments as well. Paradoxically, pseudoscience is a sign of health, not disease.

Shadows are also an inevitable consequence of light. Carl Sagan and other anti-Velikovskians believed that greater scientific literacy could \”cure\” the ill of pseudoscience. Don\’t get me wrong—scientific literacy is a wonderful thing, and I am committed to expanding it. But it won\’t eradicate the fringe, and it won\’t prevent the proliferation of doctrines the scientific community decries as pseudoscience.

Nevertheless, something needs to be done. Demarcation may be an activity without rules, a historically fluctuating marker of the worries of the scientific community, but it is also absolutely vital. Not everything can or should be taught in science courses in school. Not every research proposal can or should receive funds. When individuals spread falsehood and misinformation, they must be exposed.

We can sensibly build science policy only upon the consensus of the scientific community. This is not a bright line, but it is the only line we have. As a result, we need to be careful about demarcation, to notice how we do it and why we do it, and stop striving for a goal of universal eradication of the fringe that is frankly impossible. We need to learn what we are talking about when we talk about pseudoscience.

Michael D. Gordin is a professor of history at Princeton University. He is the author of The Pseudoscience Wars: Immanuel Velikovsky and the Birth of the Modern Fringe, due out in October from the University of Chicago Press.