Philosophers of science are not known for agreeing with each other—contrariness is part of the job description. But for thousands of years, from Aristotle to Thomas Kuhn, those who study what science is have roughly categorized themselves into two basic camps: “realists” and “anti-realists.”

In philosophical terms, “anti-realists” or “empiricists” understand science as investigating the properties of observable objects via experiments. Empirical theories are constrained by the experimental results. “Realists,” on the other hand, speculate more freely about the possible shape of the unobservable world, often designing mathematical explanations that cannot (yet) be tested. Isaac Newton was a realist, as are string theorists.

DISENCHANTED FOREST: Philosopher Bas C. van Fraassen (above) has likened the unobservable world to an “insidiously enchanted forest,” where scientists can get lost in a thicket of metaphysical speculation.Courtesy of Bas van Fraassen

Most scientists do not lose sleep worrying about philosophical divides. But maybe they should; Albert Einstein certainly did, as did Niels Bohr, and Erwin Schrödinger. In the 20th century, Kuhn’s cataloguing of the “paradigmatic” nature of scientific revolutions entered the scientific consciousness. As did Karl Popper’s requirement that only theories that can in principle be determined to be false are scientific. “God exists,” for example, is not falsifiable.

But outside the halls of the academy, the influential works of philosophers of science, such as Rudolf Carnap, Wilfrid Sellars, Paul Feyerabend, and Bas C. van Fraassen, to list but a few, are little known to many scientists and the public.

As the inventor of “constructive empiricism,” van Fraassen is widely acknowledged by his peers as one of the greatest living philosophers. (He calls himself “a philosopher’s philosopher.”) Van Fraassen does not write for the philosophically uninitiated, but his books are in no danger of going out of print.

“In 1980, van Fraassen’s The Scientific Image singlehandedly changed the terms of the debate between scientific realism and empiricism,” says Jos Uffink of the University of Minnesota. “He rescued empiricism from the dead end of logical positivism.”

In his 2008 book, Scientific Representation: Paradoxes of Perspective, van Fraassen argued that experimental data is nothing more nor less than a representation of an observable fragment of a fundamentally unobservable universe. He argued that while it is scientifically acceptable to believe that data represents a physical state of an “it,” that does not necessarily mean “it” exists.

As the ineluctably empiricist philosopher, Ludwig Wittgenstein, quipped, “Whereof one cannot speak, thereof one must be silent.” And yet many scientists speak of unobservables as if they are embedded in a map of reality that can be discovered.

It is practically impossible to describe the chaos of what actually happens in the world.

Constructive empiricism does not allow an empiricist to project truth onto the unobservable world—which van Fraassen likened to an “insidiously enchanted forest” in Scientific Representation. In the history of science, the enchanted forest has successively been populated with rain gods, musical spheres, phlogiston, ether, multiple universes, big bangs, cosmic inflation, dark matter, dark energy, and singularities. A scientist who believes in the existence of these unobserved entities has wandered into a thicket of metaphysical speculation, and left the realm of science, says van Fraassen.

Science is walled off from metaphysics in van Fraassen’s brand of empiricism by the demand that experimental data must correlate with at least part of the structure of a theoretical model. His bedrock notion of “empirical adequacy” stops at that, forbidding itself to speculate about the (metaphysical) nature of unobserved phenomena.

Fortunately, constructive empiricism allows science to proceed without providing an ontological map of the whole shebang. By way of example, there is evidence for what goes on inside a proton, but that does not allow us to assume the existence of quarks. Tons of data from linear accelerators fit into an empirically adequate model of what quarks might be. But to claim that quarks exist is a metaphysical, not a scientific statement.

Nautilus caught up with van Fraassen in July at a conference on “Quantum Interaction” at San Francisco State University.

You most often talk about empiricism in the language of physics. Does the typical physicist give a darn about philosophy?

Physicists working in quantum optics, superconductivity, and particle physics do not generally have much patience for philosophical logistics. That is a change from the turn of the last century: Max Planck, Ernst Mach, and Einstein were classically educated; they wrangled over the boundary separating empiricism from metaphysics. But there is a subgroup of physicists working on foundational problems in cosmology and quantum mechanics, and they talk to philosophers out of necessity.

What led you to the philosopher’s life?

I was born during the Nazi occupation of the Netherlands. My father was a steam fitter. The Germans transported him to Hamburg to work in a factory. He escaped once, but was captured and threatened with incarceration in a concentration camp. After the war, he made his way back to us, and we immigrated to Edmonton, Alberta, in western Canada.

In high school, I read most of the books in the 200 Dewey Decimal section of the town library: religion, psychology, and philosophy. I devoured the post-war Existentialists. Sartre. Camus. Their philosophical worldview was shaped by the war, as was mine.

While an undergraduate in Canada, I decided to become a professional philosopher. The logical empiricist, Hans Reichenbach, was my first intellectual hero. His “Berlin Circle” of empiricists in the early 1930s included Einstein, whose mathematical model of special relativity dovetailed with the observable phenomena, without trying to explain the whys.

Einstein’s work illuminated an empirical path away from a 17th-century type of metaphysical realism, the philosophical approach that claims truth for unobservable phenomena. Early 20th-century empiricists flirted with logical positivism, but that approach fell into disrepute; it was viewed as denying the existence of an objective reality.

You were accepted as a graduate student at Harvard and Cambridge. Why did you choose the University of Pittsburgh?

In the early 1960s, Pittsburgh was a hotbed of debate in the philosophy of science. I studied with the best, including Adolf Grünbaum, Wilfrid Sellars, and I studied the work of Quine and Feyerabend. A backlash against the excessive anti-realism of the positivists, such as Carnap, strengthened the position of the realists, including my dissertation director Grünbaum, who viewed spacetime as a real physical object, even though it could not be directly observed. I resisted adopting the realist stance. But I did not know how to refute the main realist assertion, that science can explain what happens beyond the realm of experiment.

Did you experience an epiphany?

In 1975, I took a sabbatical from teaching at Yale. Traveling with a car and tent, I camped in southern Europe, Romania, Turkey, and North Africa. I lugged around books and photocopies of philosophical papers by my heroes. Around campfires, I hand wrote the papers that later became chapters in The Scientific Image.

My main point is that it is practically impossible to describe the chaos of what actually happens in the world. We can construct useful theories or models that are empirically adequate—that tell us something, for instance, about the behavior of what we call electrons, without having to say what an electron is. Parts of a theoretical model can be judged as true or false, based upon the reproducibility of the data. But, to be useful, to be empirically adequate, the data does not have to fit into some overarching theory about the organization of the world.

Science is a large scale, human enterprise and we need boundaries to determine what we can say is true or not about the world around us. Empiricism is a stance, a pragmatic attitude that is self-constrained by what I call “bridled irrationality.” That means that the data itself restricts what is rational to believe about the world; it creates a boundary.

EARLY INFLUENCE: In high school, van Faassen says, “I devoured the post-war Existentialists. Sartre. Camus. Their philosophical worldview was shaped by the war, as was mine.” Jean-Paul Sartre in Venice in 1951 (above).Wikimedia

Is there an objective reality?

It is a matter of fact whether or not electrons are real. The physical world is certainly real; it objectively exists, even though we cannot glimpse more than a tiny part of it. It is the role of science to make predictive theories about phenomena which we can observe, not what we cannot observe. We will never see the particle itself, only its representations, its images, but we strive to collect a body of data that enables a theory to predict what objects do.

The role of science is to create theories that are useful in making predictions about the observable world.

If experiments reveal data that fits into part of a theoretical explanation, that fact is all that one can claim as fact; it goes beyond science to claim that the entire theoretical explanation is even “likely” to be true. Constructive empiricists see as much value in mathematical abstractions as do Neo-Platonists or idealists, but we do not agree that mathematical models must all have their counterpart in reality.

Shortly after The Scientific Image was published, I received a letter from a philosopher in New York asking me if I believe that electrons are real. I replied that such a question is not relevant to my philosophical position. He wrote back, angrily, “I am one of your readers, I have a right to know!” I did not answer him immediately. Then I saw an article by him in a popular magazine advocating torture power for the New York police. I thought, “Oh, I better not take this discussion any further.”

Do scientists have a psychological need for causal explanations?

In principle, we could develop mathematical theories on the basis of cold logic, but the desire to link causes and effects pushes science forward. University of California psychologist Alison Gopnik has published a terrific paper, “Explanation as Orgasm.” She notes that the reproduction of human life is possible without the aid of sexual orgasms, but the pursuit of orgasms noticeably results in producing children. She metaphorically compares the experience of having orgasms to what occurs to a scientist who births a theory that explains the previously hidden causes of interactions; the scientist’s desire for explanation noticeably results in producing new theories.

Is realism irrational, more orgasmic than scientific?

Realism is also a stance, but, counter the empiricist, a realist is not necessarily constrained by the facts revealed by data. The role of science is not to interpret or explain a greater reality, but to create theories that are useful in making predictions about the observable world. The sole criterion of scientific success is empirical success. Theories survive by latching onto regularities in nature.

What is an example of an empirically useful theory that relies on unobservable entities?

Is meteorology useful when it accurately predicts the weather? Of course, it is. At an earlier stage, practical meteorology, like practical medicine, was purely empirical. But today meteorology is based on classical, Newtonian physics that postulates the existence of unobservable forces that can act instantly over great distances. However, whether those forces actually exist is not relevant to the empirical success of meteorology, nor to Newtonian gravitational theory.

We used to say that the winds were created by the gods. Now we say that wind is the result of an energy differential. What is energy?

“Energy” is a theoretical term which has a mathematical meaning. It is not necessary to claim that there is an element of reality that corresponds to “energy” in order to study energetic interactions. Often people think that scientific realism presupposes a belief that unobservable objects and forces exist independently of observation. As a constructive empiricist, I am not asserting the opposite. What I am saying is that it does not matter whether the energy is real or not.

What is the philosophical criterion for scientific success?

The empiricist and the scientific realist answer that question differently. The constructive empiricist says that experimental results and measurement results are the only “real” phenomena that a scientist can witness. The criterion of success is fitting the experimental data into theoretical models that predict the data itself.

By way of example, fitting data about Higgs bosons as imaged by the Large Hadron Collider to the predictions of the Standard Model does not mean the Standard Model is a true theory, just that the Higgs data corresponds to a piece of the theory which was not previously found to be empirically adequate.

The realist disagrees, “No! Empirical adequacy does not go far enough. The criterion of scientific success is that a theory has to be entirely true.”

Moving beyond the terms of the purely philosophical debate, we need to ask ourselves, “What is the criterion for success in actual scientific practice?” Do working scientists reject a theory that is empirically adequate because they cannot believe it to be true? Because it does not fit into their preconceived ontology? Because the physics of the universe must be intelligible to humans?

To be clear, scientists tend to be pragmatists, not philosophers. The takeaway is that if the data conforms to a part of a theoretical scheme that strives to explain the structure of a chair or of the universe, that model can be used as a basis for designing more experiments. If the data does not fit the predictions of the model, then the theory is not useful for science, but fine for metaphysics, if that is what you want to do.

PARTICLE IMAGE: “We will never see the particle itself, only its representations, its images,” says van Faassen. “But we strive to collect a body of data that enables a theory to predict what objects do.” A led-ion collision recorded at the Large Hadron Collider at CERN (above).Pcharito / Wikipedia

What is the meaning of phrase one often hears, especially in the foundations of quantum mechanics, that theories can be “under-determined”?

Under-determination occurs when the data serving an empirically adequate model can fit different pictures of what is happening in the unobservable realm. That means that the same dataset can be explained by more than one kind of “real” world, which confuses the realists who want a single picture.

In quantum mechanics, the Copenhagen Interpretation espoused by Niels Bohr, David Bohm’s pilot wave interpretation, and the “relative state” interpretation of Hugh Everett III, all yield empirically adequate models that fit, and will fit, the same experimental data. They each make the same quantum mechanical predictions. But each of these interpretations posits a vastly different conception of the unobservable universe—and it is not possible to say which is “right.”

He wrote back, angrily, “I am one of your readers, I have a right to know!”

Niels Bohr’s version of the Copenhagen Interpretation forbids mixing the classical (macroscopic) and quantum (microscopic) worlds. Bohm envisages a fundamentally classical universe that subsumes quantum mechanics. Everett’s “many worlds” are totally quantum mechanical and perspectival upon the location of an observer, herself describable as a probability distribution.

Realists can hold on to one or another of these interpretative theories. While the predictive power of these theories remains the same, the realists may hope that new experimental data will one day emerge—data which quantum theory cannot accommodate as it stands. Rival interpretations may bite the dust, because they cannot accommodate the new data, while their own can be modified to meet the new challenge. In the meantime, these types of interpretive theories are metaphysical and, in my view, go well beyond the science. On the other hand, interpretations can be useful; we gain a deeper understanding of what is at stake by reflecting on rival interpretations—so, the more interpretations the better!

Are words like “determinism” and “indeterminism” useful to science?

Yes, they are, and I think that was historically a surprise. In the 19th century, many were convinced that a scientific theory is not complete if it does not offer a deterministic account of the phenomena. Quantum mechanics changed all that, first because it came with irreducible probabilities, and then definitively with the Aspect experiments that violated Bell’s inequalities. [In 1982, the French physicist Alain Aspect performed an historic series of experiments on photons that validated a controversial theorem proposed in 1964 by CERN physicist John Stewart Bell. Bell’s Theorem was a remarkably simple arithmetical proof that quantum objects can be “entangled” over great distances, correlating instantaneously to changes in each other as if they composed a single system even when separated by light-years. Entanglement is often called “spooky action at a distance.”]

There is still a respect in which quantum mechanics is deterministic. The overall quantum mechanical state or the “wave function” of the whole universe evolves deterministically: what it is now determines what will be for all time. I call this the “Go home to Mother principle.” But Mother operates behind the scenes, so to speak. On the level of observable phenomena linked to the quantum state by Born’s rule, both the data and predictions based on those data are statistical distributions. To read more into it is metaphysics. [Born’s rule is a formula for extracting classical probabilities from quantum mechanical data, giving the probability that a particle will be located at one position or another when measured.]

According to committed empiricists, how do scientists find new theories?

Leaps of faith. You make up your mind to believe an idea and then you test it, deductively. You test the premises of the theory with experiments. If the premises are true, then the conclusion will be true.

Can science explain religion and spirituality?

Some people try to incorporate religion and science by saying, “Just add a Creator to evolution.” That is a total category mistake, pseudoscience. It is not what faith is all about. It is difficult for a religious person to convey the meaning of faith. Spirituality perceives what is happening around us in a way that science cannot and is not intended to see.

Your hobby is rock climbing. Why?

Because I could die in an instant, my mind becomes wonderfully focused, but not on philosophy. Fear of falling to my death gives me a break.

Peter Byrne is an investigative reporter and science writer located in Northern California. His reporting has appeared in dozens of publications, including Scientific American, Quanta, Mother Jones, Point Reyes Light, and Salon. Byrne hangs out at

Lead collage credit: Pcharito / Wikipedia