What is the silliest claim ever made? The competition is fierce, but I think the answer is easy. Some people have denied the existence of consciousness: conscious experience, the subjective character of experience, the “what-it-is-like” of experience. Next to this denial—I’ll call it “the Denial”—every known religious belief is only a little less sensible than the belief that grass is green.

The Denial began in the twentieth century and continues today in a few pockets of philosophy and psychology and, now, information technology. It had two main causes: the rise of the behaviorist approach in psychology, and the naturalistic approach in philosophy. These were good things in their way, but they spiraled out of control and gave birth to the Great Silliness. I want to consider these main causes first, and then say something rather gloomy about a third, deeper, darker cause. But before that, I need to comment on what is being denied—consciousness, conscious experience, experience for short.

What is it? Anyone who has ever seen or heard or smelled anything knows what it is; anyone who has ever been in pain, or felt hungry or hot or cold or remorseful, dismayed, uncertain, or sleepy, or has suddenly remembered a missed appointment. All these things involve what are sometimes called “qualia”—that is to say, different types or qualities of conscious experience. What I am calling the Denial is the denial that anyone has ever really had any of these experiences.

Perhaps it’s not surprising that most Deniers deny that they’re Deniers. “Of course, we agree that consciousness or experience exists,” they say—but when they say this they mean something that specifically excludes qualia.

Who are the Deniers? I have in mind—at least—those who fully subscribe to something called “philosophical behaviorism” as well as those who fully subscribe to something called “functionalism” in the philosophy of mind. Few have been fully explicit in their denial, but among those who have been, we find Brian Farrell, Paul Feyerabend, Richard Rorty, and the generally admirable Daniel Dennett. Ned Block once remarked that Dennett’s attempt to fit consciousness or “qualia” into his theory of reality “has the relation to qualia that the US Air Force had to so many Vietnamese villages: he destroys qualia in order to save them.”

One of the strangest things the Deniers say is that although it seems that there is conscious experience, there isn’t really any conscious experience: the seeming is, in fact, an illusion. The trouble with this is that any such illusion is already and necessarily an actual instance of the thing said to be an illusion. Suppose you’re hypnotized to feel intense pain. Someone may say that you’re not really in pain, that the pain is illusory, because you haven’t really suffered any bodily damage. But to seem to feel pain is to be in pain. It’s not possible here to open up a gap between appearance and reality, between what is and what seems.

Some people not only deny the existence of consciousness; they also claim not to know what is being presumed to exist. Block responds to these deniers by quoting the reply Louis Armstrong is said to have given to those who asked him what jazz was (some people credit Fats Waller): “If you got to ask, you ain’t never gonna get to know.” Another response is almost as good, although it’s condemned by some who follow Wittgenstein. If someone asks what conscious experience is, you say, “You know what is from your own case.” (You can add, “Here’s an example,” and give them a sharp kick.) When it comes to conscious experience, there’s a rock-bottom sense in which we’re fully acquainted with it just in having it. The having is the knowing. So when people say that consciousness is a mystery, they’re wrong—because we know what it is. It’s the most familiar thing there is—however hard it is to put into words.

What people often mean when they say that consciousness is a mystery is that it’s mysterious how consciousness can be simply a matter of physical goings-on in the brain. But here, they make a Very Large Mistake, in Winnie-the-Pooh’s terminology—the mistake of thinking that we know enough about the physical components of the brain to have good reason to think that these components can’t, on their own, account for the existence of consciousness. We don’t.

*

The first cause of the Denial, behaviorism, took off about a hundred years ago as a methodological research program in experimental psychology. Psychologists had found that they couldn’t properly study consciousness because the data provided by introspection were irremediably vague. In order to be a proper science, psychology had to stick to publicly observable behavioral phenomena that are precisely measurable. The foundational text is generally agreed to be John Watson’s 1913 paper “Psychology as the Behaviorist Views It.”

Methodological behaviorism was a good and fruitful idea. For a few years, all went well. Then philosophers came on the scene, and morphed a methodology into a metaphysics. They took moderate methodological behaviorism, which puts consciousness aside and limits the scientific study of mind to behavior, and blew it up into mad metaphysical behaviorism that claims consciousness is nothing more than behavior and dispositions to behavior. As the philosopher C.D. Broad put it in 1925, this is a form of “reductive materialism.”

Proponents of this view insist that their position does not eliminate consciousness, but instead reduces it to something else. They’re right, formally speaking: to reduce X to Y isn’t to say that X doesn’t exist. It’s simply to say that X is “really just” Y, that X is “nothing more than” Y, that X is “nothing over and above” Y. And since Y is assumed to exist, X is also held to exist. For although X is nothing more than Y, it’s also nothing less than Y. When you reduce chemical processes to physical processes, you don’t deny that chemical processes exist.

All true. And yet, to reduce consciousness to behavior and dispositions to behavior is to eliminate it. To say that consciousness is really nothing more than (dispositions to) behavior is to say that it doesn’t exist. Reductionists may continue to deny this, or claim that it begs the question—that it assumes the truth of the conclusion for which it’s arguing. Formally speaking, it does beg the question, and begging the question is a well-known theoretical sin. Sometimes, however, it is the correct response.

To see this, it helps to compare the behaviorists’ reductionist theory of consciousness with the Pizza-ists’ reductionist theory of consciousness: that consciousness is really just pizza. Formally speaking, the Pizza Theory fully allows that consciousness exists, for pizza certainly exists. So, too, philosophical behaviorism fully allows that consciousness exists, because behavior certainly exists. But to say that experience is just pizza is to deny that consciousness exists, for we know that conscious experience exists, we know what it is like, and we know that it isn’t just pizza. So, too, for the claim that consciousness is just behavior.

This, then, is philosophical behaviorism, the first main version of the Denial. It was already stirring when Russell published The Analysis of Mind (1921), and was clearly on the table when Broad excoriated it in The Mind and its Place in Nature (1925), worrying that he might “be accused of breaking a butterfly on a wheel.” It may be that relatively few psychologists fell into outright philosophical behaviorism, but there was cross-infection. In 1923, the psychologist Karl Lashley aimed “to show that the statement, ‘I am conscious’ does not mean anything more than the statement that ‘such and such physiological processes are going on within me.’” Still, even an austere experimentalist like E.G. Boring, one of the leading “operationist” psychologists in the mid-twentieth century, held firmly in 1948 to the view that experience or “consciousness is what you experience immediately.”

Two years later, however, Brian Farrell judged Boring’s claim to be a “comical and pathogenic remark.” Farrell thought better times were coming. If Western societies were truly to assimilate the work of the relevant sciences, “then it is quite possible that the notion of ‘experience’ will be generally discarded as delusive.” As things are, it’s only by “restricting the use of the word ‘experience’ to ‘raw feels’ [that we can] go on defending the view that ‘experience’ and ‘behavior’ are not identical; and this line of defence is hopeless.” In the present state of our language, “the notion of ‘experience’ can be shown to resemble an occult notion like ‘witchcraft’ in a primitive community that is in the process of being acculturated to the West.” Fortunately, science “is getting to the brink of rejecting [experience]… as ‘unreal’ or ‘non-existent.’”

At this point, the philosophers had left the psychologists in the dust in the race to folly. Farrell’s thoughts were echoed by, among others, the radical philosopher of science Paul Feyerabend (1962) and Richard Rorty (1965); and they were influential in the vast upsurge of discussion of consciousness that followed the publication of the psychologist Ullin Place’s paper “Is Consciousness a Brain Process?” (1956), and the Australian philosopher Jack Smart’s paper “Sensations and Brain Processes” (1959). But, by now, something else was in play. For philosophers were not—or not primarily—motivated by behaviorist considerations in their denial of the existence of consciousness. Their line of thought was, in one striking respect, far worse. For it does at least follow from philosophical behaviorism that consciousness doesn’t really exist, whereas these philosophers were motivated by something—a commitment to naturalism—from which it doesn’t even begin to follow that consciousness doesn’t exist.

*

Naturalism states that everything that concretely exists is entirely natural; nothing supernatural or otherwise non-natural exists. Given that we know that conscious experience exists, we must as naturalists suppose that it’s wholly natural. And given that we’re specifically materialist or physicalist naturalists (as almost all naturalists are), we must take it that conscious experience is wholly material or physical. And so we should, because it’s beyond reasonable doubt that experience—what W.V. Quine called “experience in all its richness… the heady luxuriance of experience” of color and sound and smell—is wholly a matter of neural goings-on: wholly natural and wholly physical.

It’s true that we can’t understand how experience can be wholly a matter of neural goings-on, when we start out from the way the brain appears to physics or neurophysiology. Crucially, though, there’s no reason to give the way the brain appears to physics or neurophysiology priority over the way it appears to the person having the experience. Rather the reverse, as Russell pointed out as early as 1927: he annoyed many, and incurred some ridicule, when he proposed that it was only the having of conscious experience that gives us any insight into the intrinsic nature of the stuff of the brain. His point was simple: first, we know something fundamental about the essential nature of conscious experience just in having it; and second, conscious experience is literally part of the physical stuff of the brain, if materialism is true.

Genuine naturalists, then, are outright realists about consciousness, who accept that they are, in many ways, profoundly ignorant of the fundamental nature of the physical. They understand the respect in which the great naturalistic project, spearheaded by physics, hasn’t decreased our ignorance, but increased it—precisely because of its advances and successes. We don’t understand quantum mechanics, or “dark energy,” or “dark matter,” or a host of other things. So be it.

But then—in the middle of the twentieth century—something extraordinary happens. Members of a small but influential group of analytic philosophers come to think that true naturalistic materialism rules out realism about consciousness. They duly conclude that consciousness doesn’t exist. They reach this conclusion in spite of the fact that conscious experience is a wholly natural phenomenon, whose existence is more certain than any other natural phenomenon, and with which we’re directly acquainted, at least in certain fundamental respects. These philosophers thus endorse the Denial.

The problem is not that they take naturalism to entail materialism—they’re right to do so. The problem is that they endorse the claim that conscious experience can’t possibly be wholly physical. They think they know this, although genuine naturalism doesn’t warrant it in any way. So they, like the behaviorists, claim that consciousness doesn’t exist, although many of them conceal this by using the word “consciousness” in a way that omits the central feature of consciousness—the qualia, the “heady luxuriance.”

The situation grows stranger when one reflects that almost all their materialist forebears, stretching back over 2,000 years to Leucippus and Democritus, completely reject the view that experience can’t be physical, and hold instead (as all serious materialists must) that experience is wholly physical. Russell made the key observation in 1927: “We do not know enough of the intrinsic character of events outside us to say whether it does or does not differ from that of ‘mental’ events”—whose nature we do know. He never wavered from this point. In 1948, he noted that physics simply can’t tell us “whether the physical world is, or is not, different in intrinsic character from the world of mind.” In 1956, he remarked that “we know nothing about the intrinsic quality of physical events except when these are mental events that we directly experience.” But the Deniers weren’t listening, and they still aren’t.

Why do the Deniers ignore a long line of distinguished materialist predecessors and ally themselves with Descartes, their sworn enemy, in holding that experience can’t possibly be physical—thereby obliging themselves to endorse the Denial? The answer appears to be that they share with Descartes one very large assumption: that we know enough about the physical to be certain that experience can’t be physical.

It’s easy to see how, in Descartes’s day, these two assumptions might have seemed plainly right. Matter, according to the “corpuscularian” mechanics of the day, consisted of little particles of various shapes bumping into and hooking up with each other in various ways. There was nothing more to it, and it seemed evident that it couldn’t possibly be, or account for, conscious experience. The intuition seems more excusable then than today, when quantum field theory has done away with the gritty particles of the past.

The Cartesians, then, “established it as a principle that we are perfectly acquainted with the essence of matter,” as Hume put it in 1738. This was a great mistake, and 250 years later, the leading materialist philosopher David Lewis made the same mistake, claiming “that the physical nature of ordinary matter under mild conditions is very well understood.” True, this isn’t a claim of perfect acquaintance, but it is a version of the Cartesian view, and it is assumed to justify the claim that we know enough about the physical to know that experience can’t be physical. For naturalistic materialists the conclusion follows immediately and inexorably: consciousness doesn’t really exist.

One of the strangest things about the spread of the naturalism-based Denial in the second half of the twentieth century is that it involved overlooking a point about physics that was once a commonplace, and which I call “the silence of physics.” Physics is magnificent: many of its claims are either straightforwardly true or very good approximations to truth. But all of its claims about the physical are expressed by statements of number or equations. They’re truths about quantities and relational structures instantiated in concrete reality; and these truths tell us nothing at all about the ultimate nature of the stuff of reality, the stuff that has the structure that physics analyzes. Here is Russell again (in 1948): “the physical world is only known as regards certain abstract features of its space-time structure… we know nothing about the events that make matter, except their space-time structure.” Stephen Hawking agrees in 1988: physics is “just a set of rules and equations,” which leaves open the question “what… breathes fire into the equations and makes a universe for them to describe.” Physics has nothing to say about things that can’t be expressed in general rules and equations.

This is the silence of physics—a simple point that destroys the position of many of those today who, covertly or overtly, endorse the Denial. When we grasp the silence of physics, and ask, with Eddington, “what knowledge have we of the nature of atoms that renders it at all incongruous that they should constitute a thinking [i.e., conscious] object?” The answer is simple: none. The false naturalists appear to ignore this point. They rely instead on an imaginative picture of the physical, a picture that goes radically beyond anything that physics tells or could tell us. They are in Russell’s words “guilty, unconsciously and in spite of explicit disavowals, of a confusion in their imaginative picture” of reality. This picture is provably incorrect if materialism is indeed true because, in that case, experience is wholly physical yet excluded from the picture.

*

The facts of the Denial are before us, and we have an account of how they arose: first, from a mistaken interpretation of behaviorism; then, from a mistake about what a naturalistic outlook requires. But I believe we still lack a satisfactory explanation of the Denial as long as we lack a satisfactory explanation of how these mistakes could have been made. How could anybody have been led to something so silly as to deny the existence of conscious experience, the only general thing we know for certain exists?

The explanation is as ancient as it is simple. As Cicero says, there is “no statement so absurd that no philosopher will make it.” Descartes agrees, in 1637: “Nothing can be imagined which is too strange or incredible to have been said by some philosopher.” Thomas Reid concurs in 1785: “There is nothing so absurd which some philosophers have not maintained.” Louise Antony puts it like this in 2007: “There is… no banality so banal that no philosopher will deny it.”

Descartes adds that when it comes to speculative matters, “the scholar… will take… the more pride [in his views] the further they are from common sense… since he will have had to use so much more skill and ingenuity in trying to render them plausible.” Or as C.D. Broad says, some 300 years later: some ideas are “so preposterously silly that only very learned men could have thought of them… by a ‘silly’ theory I mean one which may be held at the time when one is talking or writing professionally, but which only an inmate of a lunatic asylum would think of carrying into daily life.”

We know that silliness happens, but we may still wonder how it is possible. Perhaps we should turn to individual psychology: it can seem exciting to hold views that seem preposterously contrary to common sense—there’s something Oedipally thrilling about it when the father is an old gentleman called Ordinary Opinion. Herbert Feigl adds another psychoanalytic note: “Scholars cathect [or invest] certain ideas so strongly and their outlook becomes so ego involved that they erect elaborate barricades of defenses, merely to protect their pet ideas from the blows (or the slower corrosive effects) of criticism.”

These observations may account for why, as Hobbes notes in 1645, “arguments seldom work on men of wit and learning when they have once engaged themselves in a contrary opinion.” Descartes is right again when he says:

It frequently happens that even when we know that something is false, we get used to hearing it, and thus gradually get into the habit of regarding it as true. Confident assertion and frequent repetition are the two ploys that are often more effective than the most weighty arguments when dealing with ordinary people or those [including philosophers] who do not examine things carefully.

This is what psychologists now call “the familiarity effect” or “mere-exposure effect.” And here, Sir Francis Bacon steps in, writing in 1620:

Once the human mind has favoured certain views, it pulls everything else into agreement with and support for them. Should they be outweighed by more powerful countervailing considerations, it either fails to notice these, or scorns them, or makes fine distinctions in order to neutralize and so reject them.

Very well, but how is it possible to deny the existence of consciousness? Russell thinks it’s the fault of philosophy. There are things that “only philosophers with a long training in absurdity could succeed in believing.” But it isn’t just philosophers, as Mark Twain notes: “There isn’t anything so grotesque or so incredible that the average human being can’t believe it.”

This is how philosophers in the twentieth century came to endorse the Denial, the silliest view ever held in the history of human thought. “When I squint just right,” Dennett writes in 2013, “it does sort of seem that consciousness must be something in addition to all the things it does for us and to us, some special private glow or here-I-am-ness that would be absent in any robot… But I’ve learned not to credit the hunch. I think it is a flat-out mistake, a failure of imagination.” His position was summarized in an interview in The New York Times: “The elusive subjective conscious experience—the redness of red, the painfulness of pain—that philosophers call qualia? Sheer illusion.” If he’s right, no one has ever really suffered, in spite of agonizing diseases, mental illness, murder, rape, famine, slavery, bereavement, torture, and genocide. And no one has ever caused anyone else pain.

This is the Great Silliness. We must hope that it doesn’t spread outside the academy, or convince some future information technologist or roboticist who has great power over our lives.

This essay is adapted from Things That Bother Me: Death, Freedom, the Self, Etc., published this week by New York Review Books.