All these theories are preposterous, but that's not my problem with them. My problem is that no conceivable experiment can confirm the theories, as most proponents reluctantly acknowledge. The strings (or membranes, or whatever) are too small to be discerned by any buildable instrument, and the parallel universes are too distant. Common sense thus persuades me that these avenues of speculation will turn out to be dead ends.

By John Horgan


John Horgan, author of The End of Science, and feisty and provocative as ever, is ready for combat with scientists in the Edge community. "I'd love to get Edgies' reaction to my OpEd piece — "In Defense of Common Sense" — in The New York Times", he writes.

Physicist Leonard Susskind, writing "In Defense of Uncommon Sense", is the first to take up Horgan's challenge (see below). Susskind notes that in "the utter strangeness of a world that the human intellect was not designed for... physicists have had no choice but to rewire themselves. Where intuition and common sense failed, they had to create new forms of intuition, mainly through the use of abstract mathematics." We've gone "out of the range of experience."

Read on.


JOHN HORGAN oversees the science writings program at the Stevens Institute of Technology. His books include The End of Science and Rational Mysticism.

John Horgan's Edge bio page

THE REALITY CLUB: Verena Huber-Dyson, Robert Provine, Spencer Reiss, Daniel Gilbert, John McCarthy, Leonard Susskind respond to John Horgan. Horgan replies.


As anyone remotely interested in science knows by now, 100 years ago Einstein wrote six papers that laid the groundwork for quantum mechanics and relativity, arguably the two most successful theories in history. To commemorate Einstein's "annus mirabilis," a coalition of physics groups has designated 2005 the World Year of Physics. The coalition's Web site lists more than 400 celebratory events, including conferences, museum exhibits, concerts, Webcasts, plays, poetry readings, a circus, a pie-eating contest and an Einstein look-alike competition.

In the midst of all this hoopla, I feel compelled to deplore one aspect of Einstein's legacy: the widespread belief that science and common sense are incompatible. In the pre-Einstein era, T. H. Huxley, aka "Darwin's bulldog," could define science as "nothing but trained and organized common sense." But quantum mechanics and relativity shattered our common-sense notions about how the world works. The theories ask us to believe that an electron can exist in more than one place at the same time, and that space and time — the I-beams of reality — are not rigid but rubbery. Impossible! And yet these sense-defying propositions have withstood a century's worth of painstaking experimental tests.

As a result, many scientists came to see common sense as an impediment to progress not only in physics but also in other fields. "What, after all, have we to show for ... common sense," the behaviorist B. F. Skinner asked, "or the insights gained through personal experience?" Elevating this outlook to the status of dogma, the British biologist Lewis Wolpert declared in his influential 1992 book "The Unnatural Nature of Science," "I would almost contend that if something fits in with common sense it almost certainly isn't science." Dr. Wolpert's view is widely shared. When I invoke common sense to defend or — more often — criticize a theory, scientists invariably roll their eyes.

Scientists' contempt for common sense has two unfortunate implications. One is that preposterousness, far from being a problem for a theory, is a measure of its profundity; hence the appeal, perhaps, of dubious propositions like multiple-personality disorders and multiple-universe theories. The other, even more insidious implication is that only scientists are really qualified to judge the work of other scientists. Needless to say, I reject that position, and not only because I'm a science journalist (who majored in English). I have also found common sense — ordinary, nonspecialized knowledge and judgment — to be indispensable for judging scientists' pronouncements, even, or especially, in the most esoteric fields.

For example, Einstein's intellectual heirs have long been obsessed with finding a single "unified" theory that can embrace quantum mechanics, which accounts for electromagnetism and the nuclear forces, and general relativity, which describes gravity. The two theories employ very different mathematical languages and describe very different worlds, one lumpy and random and the other seamless and deterministic.

The leading candidate for a unified theory holds that reality stems from tiny strings, or loops, or membranes, or something wriggling in a hyperspace consisting of 10, or 16 or 1,000 dimensions (the number depends on the variant of the theory, or the day of the week, or the theorist's ZIP code). A related set of "quantum gravity" theories postulates the existence of parallel universes — some perhaps mutant versions of our own, like "Bizarro world" in the old Superman comics — existing beyond the borders of our little cosmos. "Infinite Earths in Parallel Universes Really Exist," the normally sober Scientific American once hyperventilated on its cover.

All these theories are preposterous, but that's not my problem with them. My problem is that no conceivable experiment can confirm the theories, as most proponents reluctantly acknowledge. The strings (or membranes, or whatever) are too small to be discerned by any buildable instrument, and the parallel universes are too distant. Common sense thus persuades me that these avenues of speculation will turn out to be dead ends.

Common sense — and a little historical perspective — makes me equally skeptical of grand unified theories of the human mind. After a half-century of observing myself and my fellow humans — not to mention watching lots of TV and movies — I've concluded that as individuals we're pretty complex, variable, unpredictable creatures, whose personalities can be affected by a vast range of factors. I'm thus leery of hypotheses that trace some important aspect of our behavior to a single cause.

Two examples: The psychologist Frank Sulloway has claimed that birth order has a profound, permanent impact on personality; first-borns tend to be conformists, whereas later-borns are "rebels." And just last year, the geneticist Dean Hamer argued that human spirituality — surely one of the most complicated manifestations of our complicated selves — stems from a specific snippet of DNA. Although common sense biases me against these theories, I am still open to being persuaded on empirical grounds. But the evidence for both Dr. Sulloway's birth-order theory and Dr. Hamer's "God gene" is flimsy.

Over the past century, moreover, mind-science has been as faddish as teenage tastes in music, as one theory has yielded to another. Everything we think and do, scientists have assured us, can be explained by the Oedipal complex, or conditioned reflexes, or evolutionary adaptations, or a gene in the X chromosome, or serotonin deficits in the amygdala. Given this rapid turnover in paradigms, it's only sensible to doubt them all until the evidence for one becomes overwhelming.

Ironically, while many scientists disparage common sense, artificial-intelligence researchers have discovered just how subtle and powerful an attribute it is. Over the past few decades, researchers have programmed computers to perform certain well-defined tasks extremely well; computers can play championship chess, calculate a collision between two galaxies and juggle a million airline reservations. But computers fail miserably at simulating the ordinary, experience-based intelligence that helps ordinary humans get through ordinary days. In other words, computers lack common sense, and that's why even the smartest ones are so dumb.

Yes, common sense alone can lead us astray, and some of science's most profound insights into nature violate it; ultimately, scientific truth must be established on empirical grounds. Einstein himself once denigrated common sense as "the collection of prejudices acquired by age 18," but he retained a few basic prejudices of his own about how reality works. His remark that "God does not play dice with the universe" reflected his stubborn insistence that specific causes yield specific effects; he could never fully accept the bizarre implication of quantum mechanics that at small scales reality dissolves into a cloud of probabilities.

So far, Einstein seems to be wrong about God's aversion to games of chance, but he was right not to abandon his common-sense intuitions about reality. In those many instances when the evidence is tentative, we should not be embarrassed to call on common sense for guidance.

[Editor's Note: First published as an Op-Ed Page article in The New York Times on August 12th]

Felix Bloch Professor of Theoretical Physics, Stanford University


Leonard Susskind Responds to John Horgan

John Horgan, the man who famously declared The End of Science shortly before the two greatest cosmological discoveries since the Big Bang, has now come forth to tell us that the world's leading physicists and cognitive scientists are wasting their time. Why? Because they are substituting difficult-to-understand and often shockingly unintuitive concepts for "everyman" common sense. Whose common sense? John Horgan's (admittedly a non-scientist) I presume.

The complaint that science — particularly physics — has lost contact with common sense is hardly new. It was used against Einstein, Bohr, and Heisenberg, and even today is being used against Darwin by the right wing agents of "intelligent design." Every week I get several angry email messages containing "common sense" (no math) theories of everything from elementary particles to the rings of Saturn. The theories have names like "Rational Theory of the Phenomenons.

Modern science is difficult and often counterintuitive. Instead of bombastically ranting against this fact, Horgan should try to understand why it is so. The reasons have nothing to do with the perversity of string theorists, but rather, they have to do with the utter strangeness of a world that the human intellect was not designed for. Let me explain.

Up until the beginning of the 20th century, physics dealt with phenomena that took place on a human scale. The typical objects that humans could observe varied in the size from a bacterium to something smaller than a galaxy. Similarly, no human had ever traveled faster than a hundred miles an hour, or a experienced a gravitational field that accelerates objects more powerfully than the Earth's acceleration, a modest thirty two feet per second per second. Forces smaller than a thousandth of a pound, or bigger than a thousand pounds, were also out of the range of experience.

Evolution wired us with both hardware and software that would allow us to easily "grock" concepts like force, acceleration, and temperature, but only over the limited range that applies to our daily lives — concepts that are needed for our physical survival. But it simply did not provide us with wiring to intuit the quantum behavior of an electron, or velocities near the speed of light, or the powerful gravitational fields of black holes, or a universe that closes back on itself like the surface of the Earth. A classic example of the limitations of our neural wiring is the inability to picture more than three dimensions. Why, after all, would nature provide us with the capacity to visualize things that no living creature had ever experienced?

Physicists have had no choice but to rewire themselves. Where intuition and common sense failed, they had to create new forms of intuition, mainly through the use of abstract mathematics: Einstein's four dimensional elastic space-time; the infinite dimensional Hilbert space of quantum mechanics; the difficult mathematics of string theory; and, if necessary, multiple universes. When common sense fails, uncommon sense must be created. Of course we must use uncommon sense sensibly but we hardly need Horgan to tell us that.

In trying to understand the universe at both its smallest and biggest scales, physics and cosmology have embarked on a new age of exploration. In a sense we are attempting to cross a larger uncharted sea than ever before. Indeed, as Horgan tells us, it's a dangerous sea where one can easily lose ones way and go right off the deep end. But great scientists are, by nature, explorers. To tell them to stay within the boundaries of common sense may be like telling Columbus that if he goes more than fifty miles from shore he'll get hopelessly lost. Besides, good old common sense tells us that the Earth is flat.

Horgan also complains about the lack of common sense in cognitive science, i.e., the science of the mind. But the more psychologists and neuroscientists learn about the workings of the mind, the more it becomes absolutely clear that human cognition does not operate according to principles of common sense. That a man can mistake his wife for a hat is-well-common nonsense. But it happens. Cognitive scientists are also undergoing a rewiring process.

Finally I must take exception to Horgan's claim that "no conceivable experiment can confirm the theories [string theory and cosmological eternal inflation] as most proponents reluctantly acknowledge." Here I speak from first hand knowledge. Many, if not all, of the most distinguished theoretical physicists in the world — Steven Weinberg, Edward Witten, John Schwarz, Joseph Polchinski, Nathan Seiberg, Juan Maldacena, David Gross, Savas Dimopoulos, Andrei Linde, Renata Kallosh, among many others, most certainly acknowledge no such thing. These physicists are full of ideas about how to test modern concepts — from superstrings in the sky to supersymmetry in the lab.

Instead of dyspeptically railing against what he plainly does not understand, Horgan would do better to take a few courses in algebra, calculus, quantum mechanics, and string theory. He might then appreciate, even celebrate, the wonderful and amazing capacity of the human mind to find uncommon ways to comprehend the incomprehensible.

Computer Scientist; Artificial Intelligence Pioneer, Stanford University

John Horgan pontificates:

"But computers fail miserably at simulating the ordinary, experience-based intelligence that helps ordinary humans get through ordinary days. In other words, computers lack common sense, and that's why even the smartest ones are so dumb."

Horgan regards a lack of common sense as an intrinsic characteristic of computers; I assume he means computer programs. However, much artificial intelligence research has focussed on analyzing commonsense knowledge and reasoning. I refer to my 1959 article "Programs with common sense", my 1990 collection of articles "Formalizing common sense", Erik Mueller's forthcoming book "Commonsense reasoning", and the biennial international conferences on common sense. I fear John Horgan would find this work as distressingly technical as he finds physics. Common sense has proved a difficult scientific topic, and programs with human-level common sense have not yet been achieved. It may be another 100 years.

The AI research has identified components of commonsense knowledge and reasoning, has formalized some of them in languages of mathematical logic, and has built some of them into computer programs. Besides the logic based approach, there have been recent attempts to understand common sense as an aspect of the human nervous system.

Research on formalizing common sense physics, e.g. that objects fall when pushed off a table, are not in competition with physics as studied by physicists. Rather physics is imbedded in common sense. Thus applying Newton's F = ma requires commonsense reasoning. Physics texts and articles do not consist solely of equations but contain common sense explanations.

When Horgan says that string theory is untestable, he is ignoring even the popular science writing about string theory. This literature tells us that the current untestability of string theory is regarded by the string theorists as a blemish they hope to fix.

Psychologist, Harvard University

Horgan's Op-Ed piece is such a silly trifle that it doesn't dignify serious response. The beauty of science is that it allows us to transcend our intuitions about the world, and it provides us with methods by which we can determine which of our intuitions are right and which are not. Common sense tell us that the earth is flat, that the sun moves around it, and that the people who know the least often speak the loudest. Horgan's essay demonstrates that at least one of our common sense notions is true.

Contributing Editor, Wired Magazine

Surely Susskind is joking:

"Why, after all, would nature provide us with the capacity to visualize things that no living creature had ever experienced?"

Art? Music? Heaven? God? The Red Sox win the World Series? Science fiction, for chrissake!

Buy the man a drink! This is the kind of stuff that gives scientists a bad name.

Psychologist and Neuroscientist, University of Maryland; Author, Laughter

Hunter-Gatherers Make Poor Physicists and Cognitive Neuroscientists:
Horgan 0, Susskind 1

Horgan continues to expand his franchise that is based on the technique of assertively posing provocative and often reasonable propositions. The boldness of his assertions earns him an audience that he would not otherwise achieve. But as in The End of Science, he picks a fight that he is not prepared to win and never delivers a telling blow. Susskind effectively exploits a basic weakness in Horgan's thesis, the fallibility of common sense, especially in scientific context.

Researchers working at the frontiers of many sciences use mathematical and theoretical prostheses to expand the range of phenomena that can be studied, escaping some of the limits of their evolutionary history and its neurological endowment. The startling truth is that we live in a neurologically-generated, virtual cosmos that we are programmed to accept as the real thing. The challenge of science is to overcome the constraints of our neurological wetware and understand a physical world that we know only second-hand and incompletely. In fact, we must make an intuitive leap to accept the fact that there is a problem at all. Common sense and the brain that produces it evolved in the service of our hunter-gatherer ancestors, not physicists and cognitive neuroscientists. Unassisted, the brain of Horgan or any other member of our species is not up to task of engaging certain scientific problems.

Sensory science provides the most obvious discrepancies between the physical world and our neurological model of it. We humans evolved the capacity to detect a subset of stimuli available to us on the surface of planet Earth. Different animals with different histories differ in their absolute sensitivity to a given stimulus and in the bandwidth to with they are sensitive. And some species have modes of sensation that we lack, such as electric or magnetic fields. Each species is a theory of the environment in which it evolved and it can never completely escape the limitations of its unique evolutionary history. But the problem of sensing the physical cosmos is even more complicated, because we do not directly sense physical stimuli, but are aware of only their neurological correlates. There is not, for example, any "blue" in electromagnetic radiation, pitch of B-flat in pressure changes in the air, or sweetness in sucrose. All are neurological derivatives of the physical world, not the thing itself.

Neurological limits on thinking are probably as common as those on sensing, but they are more illusive — it's harder to think about what we can't think about than what we can't sense. A good example from physics is our difficulty in understanding the space-time continuum — our intellect fails us when we move beyond the dimensions of height, width, and depth. Other evidence of our neurological reality-generator is revealed by its malfunction in illusions, hallucinations, and dreams, or in brain damage, where the illusion of reality does not simply degrade, but often splinters and fragments in unanticipated ways.

The intellectual prostheses of mathematics, computers, and instrumentation loosen but do not free our species of the constraints of its neurological heritage. We do not build random devices to detect stimuli that we cannot conceive, but build outward from a base of knowledge. A neglected triumph of science is how far we have come with so flawed an instrument as the human brain and its sensoria. Another is in realizing the limits of common sense and its knowledge base of folk wisdom.

Logician; Emeritus Professor, University of Calgary


It seems to me that John Horgan in his Edge piece "In Defense of Common Sense" is confusing "common sense" with "prejudice". The human capacity for common sense reasoning is undergoing an evolutionary process as science and technology are progressing. Just look back over the last two millennia for spectacular illustrations of this pretty obvious observation. Presumably Mr. Horgan watches TV, uses his personal computer and takes airplanes to get places he cannot reach on foot nor by his questionably commonsensical motor car. If he does not know how to fix whatever trouble his car may come up with — like some people do — he really should not drive it.

To some of my colleagues the telescope serves as the extension of their vision to others the cloud chamber extends the reach of their cognition, just the way his car serves Mr Horgan to get around. In the cloud chamber we witness effects of events too small to see directly. Oh there are so many wonderful illustrations of this evolution of the human cognitive faculties. Ideas, models, conjectures acquiring reality by circumstantial evidence and repeated reasoning become part of our life; as they get entrenched our common sense expands through familiarity. Sometime our notions have to be adjusted, or some, like the idea of the ether, become obsolete. That too is progress.

Common sense that refuses to evolve becomes prejudice, or bigotry to use a more bold expression.

I have seen quite a bit of scientific evolution in my time. In my childhood the planetary model of the atom was the way we were thinking of matter; now it has become a metaphor or a handy tool, useful under certain conditions. The same is about to happen with strings. We have learned to think more abstractly, we do not really need to think of strings as wiggly worms much too small to see. We have become quite adept at mathematical modeling. I'd love to be around to see the evolution of cognition happening ever so much faster. Even the men in the street are keeping pace. Let us not encourage spoil-sports like Mr Horgan.


My modest defense of common sense as a guide for judging theories — particularly when empirical evidence is flimsy — has provoked a predictable shriek of outrage from Lenny Susskind. His attempt to lump me together with advocates of intelligent design is more than a little ironic, since in rebuking me he displays the self-righteous arrogance of a religious zealot damning an infidel. Moreover, as a proponent (!!) recently acknowledged in the New York Times, string theory and its offshoots are so devoid of evidence that they represent "a faith-based initiative."

Susskind urges me to "take courses in algebra, calculus, quantum mechanics, and string theory" before I mouth off further about strings. In other words, I must become a string theorist to voice an opinion about it. This assertion recalls the insistence of Freudians — another group notoriously hostile to outside criticism and complaints about testability — that only those fully indoctrinated into their mind-cult can judge it.

Susskind’s protestations to the contrary, string theory can be neither falsified nor verified by any empirical test. At best, experiments can provide only necessary but insufficient evidence for components — such as supersymmetry — of certain variants of string theory. That is why in 2002 I bet the physicist Michio Kaku $1000 that by 2020 no one will be awarded a Nobel prize for work on string theory or similar quantum-gravity theory. (I discuss the bet with Kaku, Lee Smolin, Gordon Kane, and other physicists at "Long Bet"). Would Susskind care to make a side bet?

As to the other respondents: John McCarthy merely confirms my assertion that computer programmers have failed to simulate common sense — except that McCarthy expends many more words to make his point than I do. And like Lenny Susskind, Robert Provine and Verena Huber-Dyson merely point out that many scientific theories violate popular, common-sense intuitions about nature and yet prove to be empirically correct.

No kidding. I said just that in my essay. The question that I raised — and that all these respondents have studiously avoided — is what we should do when presented with theories such as psychoanalysis or string theory, which are not only counterintuitive but also lacking in evidence. Common sense tells me that in these cases common sense can come in handy.


John Brockman, Editor and Publisher
Russell Weinberger, Associate Publisher

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