The Compatibility of Naturalism and Scientific Realism

Brian Holtz
Written Mar 2003, Updated Nov 2003

In The Incompatibility of Naturalism and Scientific Realism (1998), the U.T. Austin philosopher Robert Koons argues that "Nature is comprehensible scientifically only if nature is not a causally closed system -- only if nature is shaped by supernatural forces". Koons' fundamental mistake in his paper is to treat our epistemological criteria for truth -- parsimony and possibly other unspecified "quasi-aesthetic considerations" -- as if they were empirical conclusions rather than methodological assumptions. Koons mistakes a definitional connection for a causal connection, and thus mistakenly concludes that "scientific realism" rules out philosophical naturalism.

Koons writes:

Whenever philosophers bother to offer a defense for philosophical naturalism, they typically appeal to the authority of natural science [..] since, at present, our best scientific picture of the world is an essentially materialistic one, with no reference to causal agencies other than those that can be located within space and time.
This defense of naturalism presupposes a version of scientific realism: unless science provides us with objective truth about reality, it has no authority to dictate to us the form which our philosophical ontology and metaphysics must take.

This defense of naturalism could also be based on a parsimonious empiricism that provisionally accepts our fundamental natural laws as brute facts while rejecting ontological commitments for which we lack motivating phenomena. Science so strongly supports naturalism today not because we are more convinced than ever of its objective connection to "reality", but because there are so few phenomena left for which our natural laws cannot account. As I say in my book:

[There is a] human propensity to take any mysterious phenomenon as an indication of supernatural intentionality. Primitive humans invented supernatural explanations for:

the daily cycle of the Sun; the motions of the Moon and planets;
the seasons; rivers, currents, winds, thunder, lightning, precipitation and drought;
the genesis, design, and diversity of life; success in farming and hunting;
the human mind; evil, misfortune, disease, pestilence, war, and death.

However, the Scientific Revolution had established by the middle 1800s that physics, chemistry, astronomy, meteorology, and physiology could be understood in naturalistic terms. Supernatural explanations still seemed necessary for the origin and mechanism of life and mind, and for the origin of the universe itself. In the subsequent century, science outlined the basic answers for these questions, and theism began to be abandoned by serious thinkers.
[..]
Science in the past left vast swaths of phenomena unexplained. The darkness of infinite star-filled space was considered Olber's Paradox until well into the 20th century. The Sun was a marvel of inexplicable energy as recently as 1900. Disease and heredity and the blueness of the sky were still unexplained in 1850. Electricity and magnetism were spooky curiosities as recently as 1800. In 2000 there were still big mysteries about purposes and origins, but fewer marvels about what some phenomenon might possibly be. Perhaps humanity's biggest marvel in 2000 was quantum action at a distance, followed distantly by minor marvels like dark matter, gamma ray bursters, and high-temperature superconductivity. Even a phenomenon as marvelous as mind has been demonstrated to be neurological -- although diehard dualists insist that consciousness is a true marvel.
[..]
The most interesting phenomena in nature are mind, life, and the universe itself. The big questions of natural science seek the origin, mechanism and fate of mind, life, and the universe. Before the 1860's, humans had only the beginnings of an answer to only one of these questions. Newton and a few others had figured out part of the mechanism of the universe, but the other eight questions were answered with a combination of biblical myths and wild guesses. By the end of the 1960's, humanity had, for all nine questions, outlined answers that will probably still be considered correct in two hundred, two thousand, and two million years.

Koons continues:

Science construed as a mere instrument for manipulating experience, or merely as an autonomous construction of our society, without reference to our reality, tells us nothing about what kinds of things really exist and act. Koons says a science-based defense of naturalism assumes realism, but he here seems to assume it himself. An empiricist would instead say that experience is all we can know about "our reality" and "what kinds of things really exist and act".

1. Scientific realism
2. Ontological naturalism (the world of space and time is causally closed)
3. There exists a correct naturalistic account of knowledge and intentionality (representational naturalism)

By scientific realism, I intend a thesis that includes both a semantic and an epistemological component. Roughly speaking, scientific realism is the conjunction of the following two claims:
1. Our scientific theories and models are theories and models of the real world.
2. Scientific methods tend, in the long run, to increase our stock of real knowledge.

As shown below, Koons' argument rests on a mistaken premise that "the pervasiveness of the simplicity criterion in our scientific practices" is an empirical conclusion rather than a methodological assumption.

Ontological naturalism is the thesis nothing can have any influence on events and conditions in space and time except other events and conditions in space and time.

A brief aside is in order here concerning Koon's definition of naturalism. It doesn't quite capture what is meant by naturalism, since by his criterion an omnipotent being wouldn't be supernatural as long as all of its influences on the universe were exerted from within the universe. Naturalism isn't about causal closure in a spatiotemporal sense, so much as causal closure in an intentional sense. If Jesus had worked actual wonders but had done so merely using advanced extra-spatiotemporal technology based on laws making no reference to the volition of Yahweh or anyone else, then even Christians could no longer consider Jesus supernatural. Thus, naturalism is the thesis that no fundamental law of physics makes irreducible reference to (or was created by) any agency's volition.

According to the ontological naturalist, there are no causal influences from things 'outside' space: either there are no such things, or they have nothing to do with us and our world.

If there were causal influences 'outside' spacetime, but those influences were found to follow new laws that made no irreducible reference to any agency's volition, then people who are naturalists today would (rightly) continue to consider themselves naturalists.

(I agree with Koons that naturalism is the assertion of causal closure, and my disagreement with his characterization of that closure as spatiotemporal is independent of my disagreement with his paper's overall argument. His argument, and my response, is the same for either notion of naturalism.)

I will argue that Nature is comprehensible scientifically only if nature is not a causally closed system -- only if nature is the shaped by supernatural forces (forces beyond the scope of physical space and time). My argument requires two critical assumptions:
PS: A preference for simplicity (elegance, symmetries, invariances) is a pervasive feature of scientific practice.
ER: Reliability is an essential component of knowledge and intentionality, on any naturalistic account of these.

Simplicity, elegance, symmetry, and invariance are all just facets of parsimony (i.e. explanatory economy). Koons' fundamental mistake in his paper is to treat our epistemological criteria for truth -- parsimony and other unspecified "quasi-aesthetic considerations" -- as if they were empirical conclusions rather than methodological assumptions.

The Pervasiveness of Simplicity
Philosophers and historians of science have long recognized that quasi-aesthetic considerations, such as simplicity, symmetry, and elegance, have played a pervasive and indispensable role in theory choice. For instance, Copernicus's heliocentric model replaced the Ptolemaic system long before it had achieved a better fit with the date because of its far greater simplicity. Similarly, Newton's and Einstein's theories of gravitation won early acceptance due to their extraordinary degree of symmetry and elegance. In his recent book, Dreams of a Final Theory, physicist Steven Weinberg included a chapter entitled "Beautiful Theories", in which he detailed the indispensable role of simplicity in the recent history of physics.

Simplicity, symmetry, elegance, simplicity, symmetry, elegance, simplicity: again, all of these are facets of parsimony.

According to Weinberg, physicists use aesthetic qualities both as a way of suggesting theories and, even more importantly, as a sine qua non of viable theories. Weinberg argues that this developing sense of the aesthetics of nature has proved to be a reliable indicator of theoretical truth.

If Weinberg identifies any specific "aesthetic quality" of physical theories that cannot be considered a form of parsimony, Koons does not mention it in his paper.

As Weinberg explains, "Weirdly, although the beauty of physical theories is embodied in rigid, mathematical structures based on simple underlying principles, the structures that have this sort of beauty tend to survive even when the underlying principles are found to be wrong.... We are led to beautiful structures by physical principles, but the beauty sometimes survives when the principles themselves do not."

It's hardly surprising that mathematical principles are often applicable to more than just the particular physical theory that may have first motivated their elucidation. Indeed, the very nature of mathematics is that it abstracts from physical particulars to truths about inference, order, quantity, and relation that would hold in any universe regardless of its physics.

Weinberg notes that the simplicity that plays this central role in theoretical physics is "not the mechanical sort that can be measured by counting equations or symbols"

How does Weinberg know this? Koons does not say. Has Weinberg performed any information-theoretic analyses of various theories of physics? Has he tried to rigorously quantify the explanatory scope, the background assumptions, the evidentiary fit, the order of complexity, or the information content of these theories?

The recognition of this form of beauty requires an act of quasi-aesthetic judgment. As Weinberg observes,
"There is no logical formula that establishes a sharp dividing line between a beautiful explanatory theory and a mere list of data, but we know the difference when we see it."

Weinberg is a great physicist, but these unsupported pronouncements about the philosophy of science are not to be taken as authoritative. Indeed, if what makes Weinberg a great physicist is an extraordinary (and to him ineffable) operational dexterity with the criteria that constitute parsimony in physical theories, then his pronouncements are to be taken with an extra grain of salt. Similarly, we wouldn't treat Michael Jordan as an authority on kinesthetics.

even though we have no reason to think that the origin of our aesthetic attunement to the structure of the universe is mysteriously prior to experience, there remains the fact that experience has attuned us to something, and this something runs throughout the most fundamental laws of nature

Koons has not established that this "something" is anything other than the a priori methodological principle of parsimony.

As Weinberg concludes, "It is when we study truly fundamental problems that we expect to find beautiful answers. We believe that, if we ask why the world is the way it is and then ask why that answer is the way it is, at the end of this chain of explanations we shall find a few simple principles of compelling beauty. We think this in part because our historical experience teaches us that as we look beneath the surface of things, we find more and more beauty. Plato and the neo-Platonists taught that the beauty we see in nature is a reflection of the beauty of the ultimate, the nous. For us, too, the beauty of present theories is an anticipation, a premonition, of the beauty of the final theory. And, in any case, we would not accept any theory as final unless it were beautiful."

No matter how many times (here, nine) Koons quotes Weinberg using variants of the word 'beauty', Weinberg's rapture does not constitute a convincing argument for anything other than the proposition that parsimony works (and often engenders enthusiasm in physicists). (By "works", I mean not that it yields theories whose truth can be checked independent of their simplicity. I mean that using these theories helps us achieve our goals.)

This capacity for 'premonition' of the final theory is possible only because the fundamental principles of physics share a common bias toward a specific, learnable form of simplicity.

If there is a "specific, learnable form of simplicity" that works demonstrably better in our fundamental physics than different forms of simplicity that a priori would seem equally workable, then such an empirical finding would revolutionize the philosophy of science. However, any such "form of simplicity" would probably be considered a physical principle in its own right, rather than the signature of a designer.

[..] From PS, it follows that simplicity is a reliable indicator of the truth about natural laws. [..]

Yes, but only because simplicity is built into the definition of 'truth'.

[..] Reliability means that the association between simplicity and truth cannot be coincidental. A regular, objection association must be grounded in some form of causal connection. Something must be causally responsible for the bias toward simplicity exhibited by the theoretically illuminated structure of nature. [..]

Koons seems mistaken about the role parsimony plays in determining truth. Truth is defined as logical and parsimonious consistency with evidence and with other truth. Weinberg notwithstanding, truth has not instead been empirically determined to be such, because that would imply that we have some other criterion by which to distinguish truth from falsehood. We don't.

How could it be the case that parsimony were not a reliable indicator of the truth about natural laws? That would require that there be phenomena that can be explained equally well (in terms of all criteria except parsimony) with both a simple and a complex theory, and that the complex theory nevertheless be judged true and the simple theory false. On what possible basis could that judgment ever be reached? Extra complexity in a theory is only justified if it increases explanatory scope, or increases consistency with the evidence, or increases consistency with other theories, etc. If simplicity were not a criterion for deciding among theories, then we would have no way to choose among the infinitely many complex theories that can explain any phenomenon equally as well (in terms of all criteria except parsimony) as the simplest. (An infinity of more-complex theories can trivially be generated from the simplest theory just by adding random extra propositions that are still consistent with the evidence and other theories.)

So in what possible universe could a Weinberg say, "simple/beautiful theory A explains everything that complex/ugly theory Z explains, but we see yet again that in this universe the complex/ugly theory is true while the simple/beautiful theory is false"? He could say it only in a universe with a different definition of 'truth' than ours.

Thus, Koons mistakes a definitional connection for a causal connection, and mistakenly concludes that scientific realism rules out philosophical naturalism.

if it is a mere coincidence that the laws of nature share a certain form of aesthetic beauty,

Koons here demonstrates no such sharing, beyond parsimony in the face of the extant evidence. (Of course, the laws of nature could be arbitrarily more complex/ugly, but it has not been shown that they could not be more simple/beautiful. And even if such had been shown, considerations related to modal realism and the anthropic principle might explain such a finding.) If Koons or Weinberg or anyone else could rigorously demonstrate any such shared beauty beyond parsimony, it would revolutionize epistemology. Indeed, Weinberg's sense of such shared beauty is almost certainly somewhat post hoc. We already know that the simplest Grand Unified Theories -- e.g. SU(5) -- do not explain messy facts about our universe like the number of particle generations (three). Would Weinberg exempt SU(5) from being "beautiful" on some objective basis other than the fact that the universe is more complex than that theory wants it to be?

When we use the fact that we have discovered a form of "physical simplicity" in law A as a reason for preferring theories of law B which have the same kind of simplicity, then our method is reliable only if there is some causal explanation of the repetition of this form of simplicity in nature. And this repetition necessitates a supernatural cause.

This is the core of Koons' argument, and its premise is either false or does not support his conclusion. Candidate theory B is only preferred if it has the most logical and parsimonious consistency with the evidence and with other accepted theories. If C is more consistent on this score, but B has some "aesthetic" similarity to other already-accepted theories, C and not B will nevertheless be considered true. If Koons has a counterexample, he should describe it in detail. Koons gives no such examples, only vague and rapturous "beauty" quotes from Weinberg.

The account depends on the existence of certain general features which characterize the true answers to questions of fundamental physical theory. Far from being knowable a priori, these features may well be counterintuitive to the scientifically untrained. [See David Papineau, Philosophical Naturalism , p. 166]

The relevant quote in Papineau is:

It should be emphasized that this story does not depend on any a priori notion of simplicity. To this extent the term "simplicity" is perhaps a misnomer. The account simply depends on the existence of certain general features which characterize the true answers to questions of fundamental physical theory. Far from being knowable a priori, these features may well be counterintuitive to the scientifically untrained. Thus circular motion is not especially "simple", in the relevant sense, compared to the kinds of motion that results from inverse square force laws. Discontinuity is not "simple", notwithstanding the fact that everyday experience shows us sharp boundaries between physical objects and their surroundings. Explanations in terms of observable causes is not "simple", compared with explanation by microscopic hidden mechanisms

None of Papineau's examples constitute the kind of counterexample Koons needs. Indeed, Koons would do better to point to Noether's Theorem or the Principle of Least Action, but even these miss the mark. The problem is that any objective similarity that applies to different physical laws can itself be considered a physical principle, and needn't be taken as the signature of some supernatural designer. If Koons wants to prove a designer, he's going to have to do it the hard way: through some convincing variant of the Cosmological Argument. Koons apparently has tried, but no such argument has been found generally convincing by philosophers.

As Forster and Sober put it, "In the past, the curve fitting problem has posed a dilemma: Either accept a realist interpretation of science at the price of viewing simplicity as an irreducible and a prioristic sign of truth and thereby eschew empiricism, or embrace some form of anti-realism." [..]
[Forster and Sober] do not provide us with a rationale for treating simplicity as a desideratum in the first place. Consequently, Forster and Sober do not provide us with a way of escaping the conclusion that a reliabilist conception of scientific realism entails the reliability of simplicity as an indicator of the truth.

If empiricism is the thesis that all synthetic propositions can only be known from experience, then there is no inconsistency between empiricism and "viewing simplicity as an irreducible and a prioristic sign of truth". The problem of deciding what counts as truth cannot be decided by empirical investigation, but only through a priori analysis. Koons simply does not address the position that simplicity is inseparable from the notion of truth in an irreducible and a prioristic way.

Koons does, however, address a "pragmatic account of the simplicity criterion" that claims

we favor simpler hypotheses because they are easier to represent, to make deductions from, and to use in calculations [or] are more likely (given the presence of random observational error) to be repeatedly confirmed. However, these pragmatic justifications again sidestep the central issue, that of reliability. If our reliance on simplicity is unreliable, resulting in a bias toward simplicity that is not reflected in the constitution of nature, then we cannot combine scientific realism with representational naturalism.

Thus the pragmatic account of simplicity that Koons here argues against is something like this:

X. Simplicity picks out theories that are easier for humans to represent/compute/etc. and that may thus be more useful than complex alternatives, but in doing so simplicity leads us to local maxima that do not reliably correspond to final truths about ultimate reality.

As an alternative to thesis (X), Koons effectively asserts

K. Simplicity picks out theories that reliably turn out to be true [by some criterion of truth that presumably does not include simplicity].

The problem with thesis (K) is that there is no sensible criterion of truth that doesn't include simplicity. The counterfactual above suggests that omitting simplicity from the criteria for truth leads to absurdity. To disagree with Koons one need not assert (X), but rather:

H. Always using simplicity as a truth criterion (i.e. to help decide what theories to consider true/right/accurate/reliable) is a practice that turns out to be more satisfactory/useful/pleasant than not doing so.

This criterion of satisfactoriness or utility cannot be the simplicity-independent criterion that Koons fails to identify in thesis (K). If immediate utility were the way we decided whether individual theories were true, then we might declare true the most baroque or even contradictory theories in an orgy of convenient cognitive dissonance. The criterion of utility in (H) is employed at a more fundamental level, at which we are choosing the fixed truth criterion to apply uniformly to all theories. It might be useful for me to believe I am handsome or funny or well-liked, but I should not do so if I believe at a more fundamental level in the utility of e.g. only adopting beliefs that are parsimoniously consistent with available evidence.

To the extent that the success of natural science provides support for scientific realism (in both its semantic and epistemic versions), to that extent it provides grounds for rejecting philosophical naturalism. Thus, conventional wisdom has the relationship between natural science and naturalism exactly backwards.

The relationship between natural science and naturalism remains just as the conventional wisdom has it. Koons' thesis is essentialy that it must be a supernatural coincidence that science succeeds in dealing with the "real world". The "real world" indeed tends to reward one for choosing a worldivew based on parsimonious consistency with evidence (i.e. truth), and tends to punish one for choosing otherwise. This arrangement is not a coincidence; it does not suggest a supernatural arranger. Rather, it is entirely to be expected that there be a best practice for thinkers to adopt in thinking about the world and that thinkers under selective pressure tend to adopt that practice.