Over on the Google+, I linked to an informal essay by John Norton, in which he recounts the activities of a workshop on QFT at the Center for the Philosophy of Science at the University of Pittsburgh last October. In Norton’s telling, the important conceptual divide was between those who want to study “axiomatic” QFT on the one hand, and those who want to study “heuristic” QFT on the other. Axiomatic QFT is an attempt to make everything absolutely perfectly mathematically rigorous. It is severely handicapped by the fact that it is nearly impossible to get results in QFT that are both interesting and rigorous. Heuristic QFT, on the other hand, is what the vast majority of working field theorists actually do — putting aside delicate questions of whether series converge and integrals are well defined, and instead leaping forward and attempting to match predictions to the data. Philosophers like things to be well-defined, so it’s not surprising that many of them are sympathetic to the axiomatic QFT program, tangible results be damned.

The question of whether or not the interesting parts of QFT can be made rigorous is a good one, but not one that keeps many physicists awake at night. All of the difficulty in making QFT rigorous can be traced to what happens at very short distances and very high energies. And that’s certainly important to understand. But the great insight of Ken Wilson and the effective field theory approach is that, as far as particle physics is concerned, it just doesn’t matter. Many different things can happen at high energies, and we can still get the same low-energy physics at the end of the day. So putting great intellectual effort into “doing things right” at high energies might be misplaced, at least until we actually have some data about what is going on there.

Something like that attitude is defended here by our former guest blogger David Wallace. (Hat tip to Cliff Harvey on G+.) Not the best video quality, but here is David trying to convince his philosophy colleagues to concentrate on “Lagrangian QFT,” which is essentially what Norton called “heuristic QFT,” rather than axiomatic QFT. His reasoning very much follows the Wilsonian effective field theory approach.

The concluding quote says it all:

LQFT is the most successful, precise scientific theory in human history. Insofar as philosophy of physics is about drawing conclusions about the world from our best physical theories, LQFT is the place to look.

Massimo Pigliucci has written a primer for skeptics of determinism, in part spurred by reading (and taking issue with) Alex Rosenberg’s new book The Atheist’s Guide to Reality, which I mentioned here. And Jerry Coyne responds, mostly to say that none of this amounts to “free will” over and above the laws of physics. (Which is true, even if, as I’ll mention below, quantum indeterminacy can propagate upward to classical behavior.) I wanted to give my own two cents, partly as a physicist and partly as a guy who just can’t resist giving his two cents.

Echoing Massimo’s structure, here are some talking points:

* There are probably many notions of what determinism means, but let’s distinguish two. The crucial thing is that the universe can be divided up into different moments of time. (The division will generally be highly non-unique, but that’s okay.) Then we can call “global determinism” the claim that, if we know the exact state of the whole universe at one time, the future and past are completely determined. But we can also define “local determinism” to be the claim that, if we know the exact state of some part of the universe at one time, the future and past of a certain region of the universe (the “domain of dependence”) is completely determined. Both are reasonable and relevant.

* It makes sense to be interested, as Massimo seems to be, in whether or not the one true correct ultimate set of laws of physics are deterministic or not. He argues that we don’t know, and that’s obviously right, since we don’t know what the final theory is. But that’s a rather defeatist attitude all by itself; we can look at the theories we do understand and try to draw lessons from them.

* Classical mechanics, which you might have thought was deterministic if anything was, actually has some loopholes. We can think of certain situations where more than one future obeys the equations of motion starting from the same past. This is discussed a bit in the Stanford Encyclopedia of Philosophy article on causal determinism. But I personally don’t find the examples that impressive. For one thing, they are highly non-generic; you have to work really hard to find these kinds of solutions, and they certainly aren’t stable under small perturbations. More importantly, classical mechanics isn’t right; it’s just an approximation to quantum mechanics, and these finely-tuned classical solutions would be dramatically altered by quantum effects.

* General relativity is a classical theory, so it’s also not correct, but we don’t have the final theory of quantum gravity so it’s worth a look. As Massimo points out, there are good examples in GR where traditional global determinism breaks down; naked singularities would be an example. (Basically, determinism breaks down when information can in principle “flow in” from a singularity or boundary that isn’t included in “the whole universe at one moment of time.”) We might sidestep this problem by arguing that naked singularities aren’t physical, which is quite reasonable. But there are much more benign examples, such as anti-de Sitter space — a maximally symmetric spacetime with a negative cosmological constant. This universe has no singularities, but does have a boundary at infinity, so a single moment of time only determines part of the universe, not the whole thing. On the other hand, like the classical-mechanics examples alluded to above, this seems like a technicality that can be cleared up with a slight change of definition, e.g. by imposing some simple boundary condition at infinity.

Much more importantly, these kinds of GR phenomena are very far away from our everyday lives; there’s really no relevance to discussions of free will. GR violates global determinism in the strict sense, but certainly obeys local determinism; that’s all that should be required for this kind of discussion.

* Quantum mechanics is where things get interesting. When a quantum state is happily evolving along according to the Schrödinger equation, everything is perfectly deterministic; indeed, more so than classical mechanics, because the space of states (Hilbert space) doesn’t allow for the kind of non-generic funny business that let non-deterministic classical solutions sneak in. But when we make an observation, we are unable to deterministically predict what its outcome will be. (And Bell’s theorem at least suggests that this inability is not just because we’re not smart enough; we never will be able to make such predictions.) At this point, opinions become split about whether the loss of determinism is real, or merely apparent. This is a crucial question for both physicists and philosophers, but not directly relevant for the question of free will.

The traditional (“Copenhagen”) view is that QM is truly non-deterministic, and that probability plays a central role in the measurement process when wave functions collapse. Unfortunately, this process is extremely unsatisfying, not just because it runs contrary to our philosophical prejudices but because what counts as a “measurement” and the quantum/classical split are extremely ill-defined. Almost everyone agrees we should do better, despite the fact that we still teach this approach in textbooks. Someone like Tom Banks would try to eliminate the magical process of wave function collapse, but keep probability (and thus a loss of determinism) as a central feature. There is a whole school of thought along these lines, which treats the quantum state as a device for tracking probabilities; see this excellent post by Matt Leifer for more details.

The other way to go is many-worlds, which says that the ordinary deterministic evolution of the Schrödinger equation is all that ever happens. The problem there is comporting such a claim with the reality of our experience — we see Schrödinger’s cat to be alive or dead, not ever in a live/dead superposition as QM would seem to imply. The resolution is that “we” are not described by the entire quantum state; rather, we live in one branch of the wave function, which also includes numerous other branches where different outcomes were observed. This approach (which I favor) restores determinism at the level of the fundamental equations, but sacrifices it for the observational predictions made by real observers. If I were keeping a tally, I would certainly put this one in the non-determinism camp, for anyone interested in questions of free will.

* Then there is the question of whether or not the lack of determinism in QM plays any role at all in our everyday lives. When we flip a coin or play the lottery, one might think that the relevant probabilities are “purely classical” — i.e. they stem from our lack of knowledge about the state of the muscles and nerves in my hand and the wind and the coin that is about to be flipped, but if I knew all of those things I could make a perfectly deterministic prediction about what would happen to the coin. (Indeed, a well-trained magician can flip a coin and get whatever result they want.)

This is actually a tricky problem, to which the answers aren’t clear. Yes, there may be a level of classical description in terms of a probability distribution; but where does that probability distribution come from? Physicists disagree about whether or not quantum mechanics plays a crucial role here. Since I have friends in high places, this weekend I emailed Andy Albrecht, who answered and brought David Deutsch into the conversation. They both argue — plausibly, although I’m not really qualified to pass judgment — that essentially all classical probabilities can ultimately traced down to the quantum wave function. And indeed, that this reasoning provides the only sensible basis for talking about probabilities at all! (David mentions that Lev Vaidman seems to disagree, so it’s not uncontroversial by any means.) They are both, in other words, firmly anti-Bayesian in their view on probability. A good Bayesian thinks that probabilities are always statements about our fundamental ignorance concerning what is “really” going on. Albrecht and Deutsch would argue that’s not true, probabilities are ultimately always statements about the wave function of the universe. If they’re right — and again, it looks plausible, but I need to think about it more — then QM effects are indeed of crucial importance in accounting for our inability to predict the future in the everyday world.

* I should say something about chaos, which always comes up in these discussions. In classical mechanics, even when the underlying model is perfectly deterministic, it can often be the case that a small uncertainty in our knowledge of the initial state can lead to large uncertainty in the future/past evolution. (E.g. for the tumbling of Hyperion.) This is sometimes brought up as if it causes problems for determinism: “since tiny mistakes propagate, you couldn’t realistically predict the future anyway.” This is about as irrelevant as it is possible to be irrelevant. The Laplacian viewpoint was always that if you had perfect information, you could predict the past and future. But that was always a statement of principle, not of practice. Of course, in practice, you have nowhere near enough information to make the kinds of calculation that Laplace’s vast intellect likes to do. That was perfectly obvious long before the advent of chaos theory. The correct statement is “in a classical deterministic system, with perfect information and arbitrary computing power you can predict the future in principle, but not in practice,” and that statement is completely unaltered by an understanding of chaos.

So where does that leave us? My personal suspicion is that the ultimate laws of physics will embody something like the many-worlds philosophy: the underlying laws are perfectly deterministic, but what happens along any specific history is irreducibly probabilistic. (In a better understanding of quantum gravity, our notion of “time” might be altered, and therefore our notion of “determinism” might be affected; but I suspect that there will still be some underlying equations that are rigidly obeyed.) But that’s just a suspicion, not anything worth taking to the bank. For everyday-life purposes, we can’t get around the fact that quantum mechanics makes it impossible to predict the future robustly.

Of course, this is all utterly irrelevant for questions of free will. (I’m sure Massimo knows this, but he didn’t discuss it in his blog post.) We can imagine four different possibilities: determinism + free will, indeterminism + free will, determinism + no free will, and indeterminism + no free will. All of these are logically possible, and in fact beliefs that some people actually hold! Bringing determinism into discussions of free will is a red herring.

It matters, of course, how one defines “free will.” The usual strategy in these discussions is to pick your own definition, and then argue on that basis, no matter what definition is being used by the person you’re arguing with. It’s not a strategy that advances human knowledge, but it makes for an endless string of debates.

A better question is, if we choose to think of human beings as collections of atoms and particles evolving according to the laws of physics, is such a description accurate and complete? Or is there something about human consciousness — some strong sense of “free will” — that allows us to deviate from the predictions that such a purely mechanistic model would make?

If that’s your definition of free will, then it doesn’t matter whether the laws of physics are deterministic or not — all that matters is that there are laws. If the atoms and particles that make up human beings obey those laws, there is no free will in this strong sense; if there is such a notion of free will, the laws are violated. In particular, if you want to use the lack of determinism in quantum mechanics to make room for supra-physical human volition (or, for that matter, occasional interventions by God in the course of biological evolution, as Francis Collins believes), then let’s be clear: you are not making use of the rules of quantum mechanics, you are simply violating them. Quantum mechanics doesn’t say “we don’t know what’s going to happen, but maybe our ineffable spirit energies are secretly making the choices”; it says “the probability of an outcome is the modulus squared of the quantum amplitude,” full stop. Just because there are probabilities doesn’t mean there is room for free will in that sense.

On the other hand, if you use a weak sense of free will, along the lines of “a useful theory of macroscopic human behavior models people as rational agents capable of making choices,” then free will is completely compatible with the underlying laws of physics, whether they are deterministic or not. That is the (fairly standard) compatibilist position, as defended by me in Free Will is as Real as Baseball. I would argue that this is the most useful notion of free will, the one people have in mind as they contemplate whether to go right to law school or spend a year hiking through Europe. It is not so weak as to be tautological: we could imagine a universe in which there were simple robust future boundary conditions, such that a model of rational agents would not be sufficient to describe the world. E.g. a world in which there were accurate prophesies of the future: “You will grow up to marry a handsome prince.” (Like it or not.) For better or for worse, that’s not the world we live in. What happens to you in the future is a combination of choices you make and forces well beyond your control — make the best of it!

To help understand the lay of the land, we’re very happy to host this guest post by David Wallace, a philosopher of science at Oxford. David has been one of the leaders in trying to make sense of the many-worlds interpretation of quantum mechanics, in particular the knotty problem of how to get the Born rule (“the wave function squared is the probability”) out of the this formalism. He was also a participant at our recent time conference, and the co-star of one of the videos I posted. He’s a very clear writer, and I think interested parties will get a lot out of reading this.

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Why the quantum state isn’t (straightforwardly) probabilistic

In quantum mechanics, we routinely talk about so-called “superposition states” – both at the microscopic level (“the state of the electron is a superposition of spin-up and spin-down”) and, at least in foundations of physics, at the macroscopic level (“the state of Schrodinger’s cat is a superposition of alive and dead”). Rather a large fraction of the “problem of measurement” is the problem of making sense of these superposition states, and there are basically two views. On the first (“state as physical”), the state of a physical system tells us what that system is actually, physically, like, and from that point of view, Schrodinger’s cat is seriously weird. What does it even mean to say that the cat is both alive and dead? And, if cats can be alive and dead at the same time, how come when we look at them we only see definitely-alive cats or definitely-dead cats? We can try to answer the second question by invoking some mysterious new dynamical process – a “collapse of the wave function” whereby the act of looking at half-alive, half-dead cats magically causes them to jump into alive-cat or dead-cat states – but a physical process which depends for its action on “observations”, “measurements”, even “consciousness”, doesn’t seem scientifically reputable. So people who accept the “state-as-physical” view are generally led either to try to make sense of quantum theory without collapses (that leads you to something like Everett’s many-worlds theory), or to modify or augment quantum theory so as to replace it with something scientifically less problematic.

On the second view, (“state as probability”), Schrodinger’s cat is totally unmysterious. When we say “the state of the cat is half alive, half dead”, on this view we just mean “it has a 50% probability of being alive and a 50% probability of being dead”. And the so-called collapse of the wavefunction just corresponds to us looking and finding out which it is. From this point of view, to say that the cat is in a superposition of alive and dead is no more mysterious than to say that Sean is 50% likely to be in his office and 50% likely to be at a conference.

Now, to be sure, probability is a bit philosophically mysterious. It’s not uncontroversial what it means to say that something is 50% likely to be the case. But we have a number of ways of making sense of it, and for all of them, the cat stays unmysterious. For instance, perhaps we mean that if we run the experiment many times (good luck getting that one past PETA), we’ll find that half the cats live, and half of them die. (This is the Frequentist view.) Or perhaps we mean that we, personally, know that that the cat is alive or dead but we don’t know which, and the 50% is a way of quantifying our lack of knowledge. (This is the Bayesian view.) But on either view, the weirdness of the cat still goes away.

So, it’s awfully tempting to say that we should just adopt the “state-as-probability” view, and thus get rid of the quantum weirdness. But This doesn’t work, for just as the “state-as-physical” view struggles to make sense of macroscopic superpositions, so the “state-as-probability” view founders on microscopic superpositions.

Consider, for instance, a very simple interference experiment. We split a laser beam into two beams (Beam 1 and Beam 2, say) with a half-silvered mirror. We bring the beams back together at another such mirror and allow them to interfere. The resultant light ends up being split between (say) Output Path A and Output Path B, and we see how much light ends up at each. It’s well known that we can tune the two beams to get any result we like – all the light at A, all of it at B, or anything in between. It’s also well known that if we block one of the beams, we always get the same result – half the light at A, half the light at B. And finally, it’s well known that these results persist even if we turn the laser so far down that only one photon passes through at a time.

According to quantum mechanics, we should represent the state of each photon, as it passes through the system, as a superposition of “photon in Beam 1″ and “Photon in Beam 2″. According to the “state as physical” view, this is just a strange kind of non-local state a photon is. But on the “state as probability” view, it seems to be shorthand for “the photon is either in beam 1 or beam 2, with equal probability of each”. And that can’t be correct. For if the photon is in beam 1 (and so, according to quantum physics, described by a non-superposition state, or at least not by a superposition of beam states) we know we get result A half the time, result B half the time. And if the photon is in beam 2, we also know that we get result A half the time, result B half the time. So whichever beam it’s in, we should get result A half the time and result B half the time. And of course, we don’t. So, just by elementary reasoning – I haven’t even had to talk about probabilities – we seem to rule out the “state-as-probability” rule.

Indeed, we seem to be able to see, pretty directly, that something goes down each beam. If I insert an appropriate phase factor into one of the beams – either one of the beams – I can change things from “every photon ends up at A” to “every photon ends up at B”. In other words, things happening to either beam affect physical outcomes. It’s hard at best to see how to make sense of this unless both beams are being probed by physical “stuff” on every run of the experiment. That seems pretty definitively to support the idea that the superposition is somehow physical.

There’s an interesting way of getting around the problem. We could just say that my “elementary reasoning” doesn’t actually apply to quantum theory – it’s a holdover of old, bad, classical ways of thinking about the world. We might, for instance, say that the kind of either-this-thing-happens-or-that-thing-does reasoning I was using above isn’t applicable to quantum systems. (Tom Banks, in his post, says pretty much exactly this.)

There are various ways of saying what’s problematic with this, but here’s a simple one. To make this kind of claim is to say that the “probabilities” of quantum theory don’t obey all of the rules of probability. But in that case, what makes us think that they are probabilities? They can’t be relative frequencies, for instance: it can’t be that 50% of the photons go down the left branch and 50% go down the right branch. Nor can they quantify our ignorance of which branch it goes down – because we don’t need to know which branch it goes down to know what it will do next. So to call the numbers in the superposition “probabilities” is question-begging. Better to give them their own name, and fortunately, quantum mechanics has already given us a name: amplitudes.

But once we make this move, we’ve lost everything distinctive about the “state-as-probability” view. Everyone agrees that according to quantum theory, the photon has some amplitude of being in beam A and some amplitude of being in beam B (and, indeed, that the cat has some amplitude of being alive and some amplitude of being dead); the question is, what does that mean? The “state-as-probability” view was supposed to answer, simply: it means that we don’t know everything about the photon’s (or the cat’s) state; but that now seems to have been lost. And the earlier argument that something goes down both beams remains unscathed.

Now, I’ve considered only the most straightforward kind of state-as-probability view you can think of – a view which I think is pretty decisively refuted by the facts. It’s possible to imagine subtler probabilistic theories – maybe the quantum state isn’t about the probabilities of each term in the superposition, but it’s still about the probabilities of something. But people’s expectations have generally been that the ubiquity of interference effects makes that hard to sustain, and a succession of mathematical results – from classic results like the Bell-Kochen-Specker theorem, to cutting-edge results like the recent theorem by Pusey, Barrett and Rudolph – have supported that expectation.

In fact, only one currently-discussed state-as-probability theory seems even half-way viable: the probabilities aren’t the probability of anything objective, they’re just the probabilities of measurement outcomes. Quantum theory, in other words, isn’t a theory that tells us about the world: it’s just a tool to predict the results of experiment. Views like this – which philosophers call instrumentalist – are often adopted as fall-back positions by physicists defending state-as-probability takes on quantum mechanics: Tom Banks, for instance, does exactly this in the last paragraph of his blog entry.

There’s nothing particularly quantum-mechanical about instrumentalism. It has a long and rather sorry philosophical history: most contemporary philosophers of science regard it as fairly conclusively refuted. But I think it’s easier to see what’s wrong with it just by noticing that real science just isn’t like this. According to instrumentalism, palaeontologists talk about dinosaurs so they can understand fossils, astrophysicists talk about stars so they can understand photoplates, virologists talk about viruses so they can understand NMR instruments, and particle physicists talk about the Higgs Boson so they can understand the LHC. In each case, it’s quite clear that instrumentalism is the wrong way around. Science is not “about” experiments; science is about the world, and experiments are part of its toolkit.

So we asked them to go at it, with a twist: here Tim defends the proposition that time doesn’t exist, while Julian argues that it is real. I was not the only one to conclude that these guys were just as good at arguing this side as the one they actually believed.

Well worth watching — both talks are quite brilliant, in very different ways.

Both discussants have written really good books. Rosenberg recently came out with The Atheist’s Guide to Reality: Enjoying Life Without Illusions, while I very much enjoyed Flanagan’s earlier book The Really Hard Problem: Meaning in a Natural World.

Empirically, of course, naturalists often lead very enjoying and fulfilled lives. Here’s a great profile of newly minted Laureate Brian Schmidt, in his capacity as a cook and winemaker as well as an astronomer. And here’s Bob Kirshner, writing to the NYT from Friendship, Maine, about the meaning of dark energy.

The crucial moment of our panel discussion occurred when John Haught said that he couldn’t imagine a universe without God. (Without God, the universe couldn’t exist.) It would have been more crucial if I had followed up a bit more, but I didn’t because I suck (and because time was precious).

Believing that something must be true about the world because you can’t imagine otherwise is, five hundred years into the Age of Science, not a recommended strategy for acquiring reliable knowledge. It goes back to the classic conflict of rationalism vs. empiricism. “Rationalism” sounds good — who doesn’t want to be rational? But the idea behind it is that we can reach true conclusions about the world by reason alone. We don’t ever have to leave the comfort of our living room; we can just sit around, sharing some single-malt Scotch and fine cigars, thinking really hard about the universe, and thereby achieve some real understanding. Empiricism, on the other hand, says that we should try to imagine all possible ways the world could be, and then actually go out and look at it to decide which way it really is. Rationalism is traditionally associated with Descartes, Leibniz, and Spinoza, while empiricism is associated with Locke, Berkeley, and Hume — but of course these categories never quite fit perfectly well.

The lure of rationalism is powerful, and it shows up all over the place. Leibniz proclaimed various ways the world must work, such as the Principle of Sufficient Reason. Lee Smolin uses Leibnizian arguments against string theory. Many people, such as Oxford philosopher Richard Swinburne, feel strongly that the world cannot simply be; there must be a reason for its existence. Paul Davies believes that the laws of physics cannot simply be, and require an explanation. William Lane Craig believes that infinity cannot be realized in Nature. Einstein felt that God did not play dice with the universe. At a less lofty level, people see bad things happen and feel the urge to blame someone.

But the intellectual history of the past five centuries has spoken loud and clear: the dream of rationalism is a false one. The right way to attain knowledge about the universe is ultimately empirical: we formulate all the hypotheses we can, and test them against data. (Making decisions about which hypotheses best explain the data is of course a knotty problem, but that’s for another time.) Broad a priori principles are certainly useful; they can help guide us in the task of formulating and testing hypotheses. But that’s all they do — if we get lazy and start thinking that they grant us true knowledge of the world, we’ve gone off the rails.

A common manifestation of the rationalist temptation is an insistence that a certain state of affairs cannot merely exist; it must be explained, we must find a reason for it. The truth is that, if things are a certain way, there might be a reason for it, but there might not be. Both are hypotheses that should be examined. I personally have a strong feeling that the low entropy of the early universe is an unusual situation that probably has a deeper explanation — it’s a clue pointing towards something we don’t understand about the universe. But I’m careful to distinguish that I don’t know this to be true. It’s perfectly conceivable that the universe simply is that way, and there is no deeper explanation. Ultimately the decision will be made by constructing comprehensive theories and comparing them to data, not by scientists stamping their feet and insisting that a better explanation must be found.

An inquisitive five-year-old might bombard you with an endless series of “Why?” questions. Sometimes you encounter an older version of this five-year-old; someone who, when you say “I have finally formulated a successful unification of all the laws of physics!” will insist on asking “But why is it that way?” If you say “it just is,” they will say “that’s not good enough.” That’s the point at which you are allowed to turn the tables. Just start asking, “Well why isn’t it good enough? Why do I need a deeper level of explanation for how the world is?” Not that it will actually change their attitude, but it can be personally satisfying.

Favorite targets for people insisting on deeper explanations include the existence of the universe itself (as Haught was indicating) and the particular laws of physics we observe (as Davies argues). The proper scientific attitude is to say: well, there may be a deeper explanation, or there may not. Before we go out and actually look at, the universe could very well be many things. It could be a single point. It could be a line or a plane. It could be non-existent. The universe could be a fiber bundle over a Riemannian manifold, an n-dimensional cellular automaton, a trajectory in Hilbert space obeying Schroedinger’s equation, a holographic projection of a conformal field theory, the dream of a disturbed demon, a layered collection of natural and supernatural dimensions, someone’s elaborate computer simulation, or any of a million other things. It could be unique or multiple, meaningful or intrinsically purposeless. It could be brought into existence by something outside itself, or it could be sustained by a distinct being, or it could simply be. If you personally find some of these alternatives unsatisfying, that is a matter for you and your therapist to work out; reality doesn’t care. The way we will find out the truth is not to insist that it must be one way or another; it’s to understand the likely consequences of each possibility, and line them up with what we actually observe.

You can see why a rationalist line of reasoning would be attractive to the theistically inclined. If you have God intervening in the world, you can judge it by science and it’s not a very good theory. If on the other hand God is completely separate from the universe, what’s the point? But if God is a necessary being, certainly existing but not necessarily poking into the operation of the world, you can have your theological cake without it being stolen by scientific party-crashers, if I may mix a metaphor. The problem is, there are no necessary beings. There is only what exists, and we should be open to all the possibilities.

None of this is to say that there is no room for logic or reason in understanding how the world could possibly work. “2+2=4″ is a true statement in any possible world, once we specify the definitions of “2″ and “+” and “=” and “4.” But that doesn’t mean it’s a true statement about anything that actually happens in the world. The universe might very well have been something where there weren’t two collections of two things to add together, nor sufficient computing power to perform the arithmetical operation. Once we accept some hypotheses about the world (through comparing their predictions to reality), we are allowed to use reason to draw inferences from those hypotheses. (That’s kind of what I do for a living.) But step one in that process is to be open to which sets of hypotheses are actually relevant to the real world.

The temptation of rationalism can be a hard one to resist. We human beings are not blank slates; not only do we come equipped with informal heuristics for making sense of the world we see, but we have strong desires about how the world should operate. Intellectual honesty demands that we put those desires aside, and accept the world for what it actually is, whatever that may turn out to be.

Physicists are well aware that there are different vocabularies/models/theories that we can use to describe the same underlying reality. Sometimes you might want to talk about a box of gas as a fluid with pressure and velocity, other times you might want to talk about it in terms of atoms and molecules. Philosophers and psychologists might want to talk about human beings as autonomous agents who do things for reasons, while admitting that they can also be thought of as collections of cells and tissues, or even once again as atoms and molecules. The question is: what is the relationship between these different levels? In fluid mechanics/kinetic theory things are pretty clear, but in the mind/body problem things begin to get murky. (Or at least, there are people who take great pleasure in insisting that they are murky.)

Reductionism notes that some of these descriptions are more complete, and therefore arguably more fundamental, than others. In particular, some descriptions are in terms of entities that are literally smaller than the others; atoms are smaller than neurons, which are smaller than people. The smaller-level descriptions tend to have a wider range of validity; we can imagine answering certain questions in the atomic language that we can’t answer (correctly) in the fluid language, like “what happens if we divide the box in half, and then divide that in half, and so forth a million times?” It therefore seems natural to arrange the descriptions vertically: “lower” levels refer to small-scale descriptions, while “higher” levels refer to macroscopic objects. The claim of reductionism is, depending on who you talk to, that the lower-level description is either “always more complete,” or “capable of deriving the higher-level descriptions,” or “the right way to think about things.”

The reductionist paradigm is of course heavily resisted in certain quarters. Emergentists like to argue that “more is different,” and that truly novel behaviors emerge at the higher levels. All the argument then becomes about what is meant by “truly novel.” Do you mean “you never would have guessed these behaviors, just by thinking in terms of lower levels”? If so, most reductionists would readily agree. But if you mean “these behaviors are truly independent from what goes on at the lower levels,” then they would not. It is not even really clear what that would mean.

Downward causation, as I understand it, is an attempt to give some oomph to the claim that higher levels are not simply derived from lower levels. Consider the good old mental/physical divide. A reductionist would claim that the mental can ultimately be reduced to the physical. (I’m gliding over various nuanced divisions of opinion in the two-dimensional parameter space of reductionism/physicalism, but so be it.) But an antireductionist might say: “Look, I can choose to lift up my hand and put it somewhere. That’s the mental acting on the physical, with causally efficacious outcomes. You can’t describe this in terms of the physical alone; the higher level is influencing what happens at the lower level.”

That’s downward causation; the higher levels acting causally on the lower levels. If you get spooked by mind/body issues, think of the snowflakes. Sure, they are made of water molecules that act according to atomic/molecular physics. But the shape that they end up taking is highly constrained by the macroscopic crystalline structure of the snowflake itself. That wouldn’t have been visible if you were just thinking about molecules; the macroscopic structure has influenced the dynamics of the microscopic constituents.

I’m doing my best to present this idea sympathetically, but it seems completely wrong-headed to me. As far as I can tell, a major motivation for thinking about downward causation is to preserve the autonomy of mental causation. We think of ourselves as intelligent beings who do things for reasons. We would therefore like to think of the decisions we make as causing certain things to happen in the physical world. But if the mental can be simply reduced to the physical, we might worry that this way of thinking is just wrong. There aren’t “really” mental states that cause things to happen; there are simply neurons and tissues (or atoms and forces) acting according to the laws of physics/biology. Choices and other mental phenomena are just illusions (according to this line of worry). Jerry Fodor put it most vividly:

“If it isn’t literally true that my wanting is causally responsible for my reaching, and my itching is causally responsible for my scratching, and my believing is causally responsible for my saying… if none of that is literally true, then practically everything I believe about anything is false and it’s the end of the world.”

Don’t worry, Jerry Fodor! It’s not really the end of the world.

But before explaining why, let me give a sensible argument that downward causation can’t really work. It’s called the “exclusion argument” (if I’m understanding things correctly), but physicists would simply refer to “closed sets of equations.” The point is that, when we talk about the world in terms of atoms and forces, we have a closed system — any question we can ask in those terms, can be uniquely answered in those terms. (We have the same number of equations as unknowns.) So it can’t be true that we need to account for higher-level processes to follow things at the lower level; indeed, doing so would amount to overconstraining the system, and we would generically expect no consistent solutions. This is how we know that immortal souls require violations of the known laws of physics — those laws are complete by themselves, and aren’t able to support immaterial souls surviving past the body. My language is a little different from that in the philosophy literature, but I take it that this is what’s meant by the exclusion argument.

Why isn’t it, then, the end of the world? I think there are two mistakes being made here. One is to believe that if one phenomenon can be “reduced” to a lower level, then the higher-level phenomenon isn’t “real,” it’s just an illusion. (That’s how I interpret “literally true” in Fodor’s quote.) That’s a very bad way of thinking about the relationship between different levels. This is what I tried to argue in the post about free will and baseball: just because we can think of something macroscopic in terms of its microscopic parts, doesn’t mean that macroscopic thing becomes any less real. Baseball is real, temperature is real, free will is real — all in the sense that they are useful categories for organizing the macroscopic world, whether or not these concepts are nowhere to be found in the vocabulary of fundamental physics.

The second mistake is taking the hierarchy of levels too seriously, with some on top and some on the bottom. (This is related to the previous mistake, obviously.) I would suggest that a better mental image would feature a parallelism of levels with sideways relations between them. So we have a description of a box of gas in terms of atoms and molecules, and another in terms of fluid dynamics. These models sit next to each other, and have arrows moving sideways between them to indicate the map that tells us which configurations in one correspond to which configurations in the other. Sure, one vocabulary may be “more complete” in the sense that it accurately models a wider array of physical conditions, but so what? If another (“higher-level”) description obeys its own autonomous rules of evolution — that is, if we can successfully speak of its properties and outcomes without ever making reference to the any other descriptions (as is certainly true for fluids) — then this description is just as “real” and “literally true” as any others.

I think this way of thinking gets you everything you want. You are allowed to treat mental phenomena (or whatever) as perfectly “real” and causally efficacious. You are also allowed to attempt to “derive” the dynamical rules of one description from the dynamical rules of another plus the map between them. It might be easy, or it might be hard or impossible, but succeeding wouldn’t leech any of the power from the autonomous rules of the “derived” system.

All the mess comes when people try to mix up vocabularies across different levels. You should beware of crossing the streams — total protonic reversal could result, and that would be bad. We can talk about people as animals with minds and reasons, or we can talk about them as collections of cells and tissues, or we can talk about them as collections of protons, neutrons and electrons. It’s only when you start asking “what effect do my feelings have on my protons and neutrons?” that you start getting syntax errors.

This parallelism view gets strong support from dualities in physics. One thing we’ve learned is that you can have completely different descriptions of exactly the same underlying “reality,” but it’s not that one is lower-level and the other is higher-level; they’re simply different. Autonomous vocabularies provide powerful tools for discussing different features of the world in different circumstances. Knowing that you’re made of elementary particles obeying the laws of physics doesn’t make you any less of a person.

In some ways, asking whether free will exists is a lot like asking whether time really exists. In both cases, it’s different from asking “do unicorns exist?” or “does dark matter exist?” In these examples, we are pretty clear on what the concepts are supposed to denote, and what it would mean for them to actually exist; what’s left is a matter of collecting evidence and judging its value. I take it that this is not what we mean when we ask about the existence of free will.

It’s possible to deny the existence of something while using it all the time. Julian Barbour doesn’t believe time is real, but he is perfectly capable of showing up to a meeting on time. Likewise, people who question the existence of free will don’t have any trouble making choices. (John Searle has joked that people who deny free will, when ordering at a restaurant, should say “just bring me whatever the laws of nature have determined I will get.”) Whatever it is we are asking, it’s not simply a matter of evidence.

When people make use of a concept and simultaneously deny its existence, what they typically mean is that the concept in question is nowhere to be found in some “fundamental” description of reality. Julian Barbour thinks that if we just understood the laws of physics better, “time” would disappear from our vocabulary. Likewise, discussions about the existence of free will often center on whether we really need to include such freedom as an irreducible component of reality, without which our understanding would be fundamentally incomplete.

There are people who do believe in free will in this sense; that we need to invoke a notion of free will as an essential ingredient in reality, over and above the conventional laws of nature. These are libertarians, in the metaphysical sense rather than the political-philosophy sense. They may explicitly believe that conscious creatures are governed by a blob of spirit energy that transcends materialist categories, or they can be more vague about how the free will actually manifests itself. But in either event, they believe that our freedom of choice cannot be reduced to our constituent particles evolving according to the laws of physics.

This version of free will, as anyone who reads the blog will recognize, I don’t buy at all. Within the regime of everyday life, the underlying laws of physics are completely understood. There’s a lot we don’t understand about consciousness, but none of the problems we face rise to the level that we should be tempted to distrust our basic understanding of how the atoms and forces inside our brains work. Note that it’s not really a matter of “determinism”; it’s simply a question of whether there are impersonal laws of nature at all. The fact that quantum mechanics introduces a stochastic component into physical predictions doesn’t open the door for true libertarian free will.

But I also don’t think that “playing a necessary role in every effective description of the world” is a very good way of defining “existence” or “reality.” If there is anything that modern physics has taught us, it’s that it’s very often possible to discuss a single situation in two or more completely different (but equivalent) ways. Duality in particle physics is probably the most carefully-defined example, but the same idea holds in more familiar contexts. When we talk about air in a room, we can describe it by listing the properties of each and every molecule, or we speak in coarse-grained terms about things like temperature and pressure. One description is more “fundamental,” in that its regime of validity is wider; but both have a regime of validity, and as long as we are in that regime, the relevant concepts have a perfectly good claim to “existing.” It would be silly to say that temperature isn’t “real,” just because the concept doesn’t appear in some fine-grained vocabulary.

We talk about the world using different levels of description, appropriate to the question of interest. Some levels might be thought of as “fundamental” and others as “emergent,” but they are all there. Does baseball exist? It’s nowhere to be found in the Standard Model of particle physics. But any definition of “exist” that can’t find room for baseball seems overly narrow to me. It’s true that we could take any particular example of a baseball game and choose to describe it by listing the exact quantum state of each elementary particle contained in the players and the bat and ball and the field etc. But why in the world would anyone think that is a good idea? The concept of baseball is emergent rather than fundamental, but it’s no less real for all of that.

Likewise for free will. We can be perfectly orthodox materialists and yet believe in free will, if what we mean by that is that there is a level of description that is useful in certain contexts and that includes “autonomous agents with free will” as crucial ingredients. That’s the “variety of free will worth having,” as Daniel Dennett would put it.

I’m not saying anything original — this is a well-known position, probably the majority view among contemporary philosophers. It’s a school of thought called compatibilism: see Wikipedia, or (better) the Stanford Encyclopedia of Philosophy. Free will as an emergent phenomenon can be perfectly compatible with an underlying materialist view of the world.

Of course, just because it can be compatible with the laws of nature, doesn’t mean that the concept of free will actually is the best way to talk about emergent human behaviors. (Just because I know the rules of chess doesn’t make me a grandmaster.) There are still plenty of interesting questions remaining to be clarified. At the very least, there is some kind of tension between a microscopic view in which we’re just made of particles and a macroscopic one in which we have “choices.” David Albert does a great job of articulating this tension in this short excerpt from a Bloggingheads dialogue we did some time back.

I don’t generally think that the superior wisdom one acquires via training as a physicist grants one the power to see clearly through complicated issues and make philosophical conundrums dissolve away. But this is a case where insights from physics might actually be useful. In particular, what we are faced with is the task of reconciling effective theories at different levels of description that have apparently incompatible features: the impersonal evolution of the microscopic level (whether we go all the way to atoms, or stick with genes and neurons) and the irreducible possibility of “choice” at the macroscopic level.

This kind of tension also appears in physics. Indeed, the arrow of time is a great example. The microscopic laws of physics (as far as we know) are perfectly reversible; evolution forward in time is no different from evolution backward in time. But the macroscopic world is manifestly characterized by irreversibility. That doesn’t mean that the two descriptions are incompatible, just that we have to be careful about how they fit together. In the case of irreversibility, we realize that we need an extra ingredient: the particular configuration of our universe, not just the laws of physics.

In fact, the connection goes beyond a mere analogy. If you look up arguments against compatibilism, you find something called The Consequence Argument. This is based on the “fundamental difference between the past and future” — what we do now affects the future, but it doesn’t affect the past. Earlier times are fixed, while we can still influence later times. The consequence argument points out that deterministic laws imply that the future isn’t really up for grabs; it’s determined by the present state just as surely as the past is. So we don’t really have choices about anything. (For purposes of this discussion we can ignore the question of whether the microscopic laws really are deterministic; all that really matters are that there are laws.)

The problem with this is that it mixes levels of description. If we know the exact quantum state of all of our atoms and forces, in principle Laplace’s Demon can predict our future. But we don’t know that, and we never will, and therefore who cares? What we are trying to do is to construct an effective understanding of human beings, not of electrons and nuclei. Given our lack of complete microscopic information, the question we should be asking is, “does the best theory of human beings include an element of free choice?” The reason why it might is precisely because we have different epistemic access to the past and the future. The low entropy of the past allows for the existence of “records” and “memories,” and consequently forces us to model the past as “settled.” We have no such restriction toward the future, which is why we model the future as something we can influence. From this perspective, free will is no more ruled out by the consequence argument than the Second Law of Thermodynamics is ruled out by microscopic reversibility.

None of this quite settles the question of whether “free will” is actually a crucial ingredient in the best theory of human beings we can imagine developing. I suspect it is, but I’m willing to change my mind as we learn more. The context in which it really matters is when we turn to questions of moral responsibility. Should we hold people who do bad things responsible for their actions — even if our understanding of neuroscience improves to such an extent that we can identify precisely which gene or neuron “made them do it?” (This is the focus of Eagleman’s article.)

This is a resolutely practical question — who gets thrown in jail? Criminal law has the concept of mens rea, guilty mind. We don’t find people guilty of crimes simply because they committed them; they had to be responsible, in the sense that they had the mental capacity to have known better. In other words: we have a model of human beings as rational agents, able to gather and process information, understand consequences, and make decisions. When they make the wrong ones, they deserve to be punished. People who are incapable of this kind of rationality — young children, the mentally ill — are not held responsible in the same way.

Might we someday understand the brain so well, reducing thought to a series of mechanical processes, that this model ceases to be useful? It seems possible, but unlikely. We know that air is made of molecules, but the laws of thermodynamics haven’t lost their usefulness. Thinking of the collections of atoms we call “people” as rational agents capable of making choices seems like a pretty good theory to me, likely to remain useful for a long while to come. At least, that’s what I choose to think.

Here’s the post, which I’m cross-posting below. This might be controversial, as a lot of people on my side of things will say that there’s little point in engaging with people on the other side. And admittedly, this is a subject where feelings can be pretty entrenched. But you never know — not everyone has their mind made up on every issue, and it’s good to try to explain yourself to unsympathetic audiences on occasion. That’s all I tried to do here — to explain how I think about these things, not necessarily to pick a fight or even persuade any skeptics. I tried pretty hard to be as clear and unpretentious as I can be. (Success is for you to decide.) In a world of shouting and diatribe, I remain optimistic that real communication can occasionally occur! We’ll see how it goes.

——

I wanted to thank Vincent Torley and Denyse O’Leary for the opportunity to write a guest blog post, and apologize for how long it’s taken me to do so. I’ve written an article for the forthcoming Blackwell Companion to Science and Christianity, entitled Does the Universe Need God?, in which I argued that the answer is “no.” Vincent posed a list of questions in response. After thinking about it, I decided that my answers would be more clear if I simply wrote a coherent argument, rather than addressing the questions individually.

My goal is to try to explain my own thinking to an audience that is not predisposed to agree. We can roughly break people up into two groups: naturalists such as myself, who think that the best explanation we have for the universe involves physical quantities obeying laws of Nature and nothing else; and those who believe that a better explanation can be found by invoking a powerful being/designer/creator/God. (For the sake of simplicity I’m going to use “God” to refer to this notion, but feel free to substitute the more accurate description of your choice.) Obviously there are many nuances that are being passed over by this simple distinction, but hopefully it will suffice for this moment.

The dispute between these two camps isn’t one where people often change their minds at the drop of an argument. Minds do change, in either direction — but typically after extended periods of reflection, not suddenly in response to a single killer blog post. So persuasion is not my goal here; only explanation. I’ve succeeded if an open-minded person who disagrees with me reads the post and still disagrees, but at least understands why I hold my positions. (After giving an earlier talk, one of the theologians in the audience told me that I had persuaded him — not that God didn’t exist, but that the argument from design wasn’t the way to get to Him. That sort of real-time response is more than one can generally hope for.)

What I want to do is to elaborate on some crucial aspects of how science is done that bear directly on the issues raised by my article and some of the responses to it that I’ve seen. In particular, I want to talk about simplicity, laws, openness, explanation, and clarity. This isn’t supposed to be a comprehensive treatise on the philosophy of science, nor is it especially rigorous, or anything really new — just some thoughts on issues relevant to this conversation.

I will be taking one thing for granted: that what we’re interested in doing here is science. There are many kinds of consideration that may lead people to theism or atheism that have nothing whatsoever to do with science; likewise, one may believe that there are ways of understanding the natural world that go beyond the methods of science. I have nothing to say about that right now; that’s a higher-level discussion. I’m just going to presume that we all agree that we’re trying to be the best scientists we can possibly be, and ask what that means.

With all that throat-clearing out of the way, here’s what I have to say about these five issues.

Simplicity.

Science tries to capture the world in the simplest possible description. We are fortunate that such an endeavor is sensible, in that the world we observe exhibits various regularities. If the contents and behavior of the world were completely different from point to point and moment to moment, science would be impossible. But the regularities of the world offer a tremendous simplification of description, making science possible. We don’t need to talk separately about the charge of this electron, and the charge of that electron; all electrons have the same charge.

Simplicity can be quantified by the concept of Kolmogorov complexity — roughly, the length of the shortest possible complete description of a system. It takes longer to specify some particular list of 1,000 random numbers than it does to specify “the integers from 1 to one million,” even though the latter contains more elements. The list of integers therefore has a lower Kolmogorov complexity, and we say that it’s simpler. Scientists are trying to come up with the simplest description of nature that accounts for all the data.

Note that a theory that invokes God (or any other extra-physical categories) is, all else being equal, less simple than a theory that does not. “God + the natural world” is less simple than “the natural world.” This doesn’t mean that the idea of God is automatically wrong; only that it starts out at a disadvantage as far as simplicity is concerned. A conscientious scientist could nevertheless be led to the conclusion that God plays a role in the best possible scientific description of the world. For example, it could (in some hypothetical world) turn out to be impossible to fit the data without invoking God. As Einstein put it: “It can scarcely be denied that the supreme goal of all theory is to make the irreducible basic elements as simple and as few as possible without having to surrender the adequate representation of a single datum of experience.” Alternatively, you could imagine deriving all of the physical laws from the simpler assumption that God exists. While these strategies are conceivable, in practice I don’t think they work, as should become clear.

Laws.

A “law of nature” is simply a regularity we observe in the universe. All electrons have the same charge; energy and momentum are conserved in particle interactions. A law doesn’t necessarily have to be absolute or deterministic; the Born rule of quantum mechanics states that the probability of obtaining a certain observational result is the square of the amplitude of the corresponding branch of the wave function. A law is simply a pattern we observe in nature.

As far as science is concerned, it makes no difference whether we refer to these regularities as “laws” or “patterns” or anything else. It also doesn’t matter whether we think of them as “fundamental and irreducible features of the cosmos.” They simply are; science looks for them, and finds them. Vincent asks “How can rules exist in the absence of a mind?” That is simply not a question that science is concerned with. Science wants to know how we can boil the behavior of nature down to the simplest possible rules. You might want more than that; but then you’re not doing science. He also asks why we should believe that the rules should continue to hold tomorrow, simply because they have held in the past. Again, that’s what science does. Imagining that the same basic laws will continue to hold provides a simpler fit to the data we have than imagining (for no good reason) that they will change. If you are personally unsatisfied with that attitude, that’s fine; but your dissatisfaction is not a scientific matter.

Openness.

This is probably the most important point I have to make, and follows directly on the issue of “laws” just addressed. There is a way of trying to understand the world that might roughly be called “scholastic,” which sits down and tries to reason about how the world should be. The great success of science over the last five hundred years has been made possible by throwing out that kind of thinking in favor of a different model. Namely: we think of every possible way the world could be, and then we go out and look at the world to see which is the simplest description that fits the data. Science insists that we be open to all possibilities, and let the data decide which is true.

Suppose that you are convinced that laws of nature could not exist without a guiding intelligence that formulated them and sustains them. That’s fine for you, but it’s a deeply unscientific attitude. The scientific attitude is: “We observe that there are regularities in nature. We might imagine that they are formulated and sustained by a guiding intelligence, or that they simply exist on their own. Let’s go collect data to determine which idea is a more parsimonious fit to reality.”

The primary sin a scientist can commit is to decide ahead of time that the universe must behave in certain ways. We can certainly have intuitions about what kind of behavior “makes sense” to us as scientists — theorists are guided by their intuition all the time. But the use of that intuition is to help us develop hypotheses, not to decide which hypothesis is correct. Only confrontation with data can do that.

Explanation.

Science has a complicated relationship with “Why?” questions. Sometimes it provides direct answers: Why do all electrons have the same charge? Because they are all excitations of a single underlying quantum field. But sometimes it does not: Why is there a quantum field with the properties of electrons? Well, that’s just the way it is. Which questions have sensible answers is dependent on context, and can even change as we learn new things about the universe. To Kepler, understanding why exactly five planets orbit the Sun was a question of paramount importance. These days we think of the number of planets (eight, according to the International Astronomical Union) as something of an accident.

The point, once again, is that we can’t decide ahead of time what kinds of explanations science is going to provide for us. Science looks for the simplest possible description of the world. It might be that we will eventually understand the inner workings of nature so well that we will be able to answer every conceivable “Why?” question — we will ultimately see that things simply could not have been any other way. But it is also perfectly possible that the best possible description of the world involves some number of brute facts that have no deeper explanation. This is an issue that will ultimately be decided by the conventional progress of science, not by a priori demands that the universe must explain itself to anyone’s individual satisfaction.

Clarity.

The final point I wanted to make involves the clarity of scientific hypotheses. Perhaps “unambiguity” would have been a more precise word, but it is so ugly I couldn’t bring myself to use it.

The point is that a respectable scientific theory should be formulated in terms that are so unambiguously clear that any two people, both of whom understand the theory and have the technical competence to elucidate its consequences, will always come to the same conclusion about what the theory says. This is why the best theories we have are very often cast in the form of mathematics; the rules for manipulating equations are absolutely free of ambiguity. You tell me the initial conditions of some classical mechanical system, as well as the Hamiltonian, and I will come up with the same predictions for its future evolution as absolutely anyone else wit the same information.

Earlier I mentioned that the God hypothesis could actually be simpler than a purely naturalistic theory, if one could use the idea of God to derive the observed laws of nature (or at least some other features of the universe). This isn’t idle speculation, of course; many people have taken this road. The fundamental problem, however, is that the idea of God is utterly unclear and ambiguous, as far as conventional scientific thinking is concerned.

One might object: God is simply the most perfect being conceivable, and what could be more unambiguous than that? (One possible response, not the only one.) That sounds like a clear statement, but it’s not in any sense a clear scientific theory. For that, there would have to be a set of unambiguous rules that let you go from “the most perfect being” to the laws of nature that we see around us. As I argued in my paper, this is very far from what we actually have. It is sometimes argued, for example, that God explains the small value of the vacuum energy (cosmological constant), because without that fine-tuning life would be impossible. But why does God choose this particular value? Actually it could be quite a bit larger and life would still be very possible. Why are there 100 billion galaxies in addition to the one we live in? Why are there three generations of elementary particles, when life is only constructed from the first one? Why was the entropy of the early universe enormously smaller than it needed to be to support life?

Obviously these are perfectly good questions for naturalistic theories as well as for God. The problem is that we can imagine coming up with naturalistic theories that do provide clear answers, while it’s very hard to see how God could ever do that. The problem is simple: God isn’t expressed in the form of equations. There is no clear and unambiguous map from God to a particular set of laws of physics, or a particular configuration of the universe. If there were, we would be using that map to make predictions. What does God have to say about supersymmetry, or the mass of the Higgs boson, or the amplitude of gravitational-wave perturbations of the cosmic microwave background? If we claim that God “explains” the known laws of physics, the same method of explanation should work for the unknown laws. It’s not going to happen.

It’s not clear to me that anyone who believes in God should actually want it to happen. There is a very strong tension between what scientists look for in a theory — clear and unambiguous connections between premises and predictions — and the way that religious believers typically conceive of God, as a conscious being that is irreducibly free to make choices. Does anyone really want to reduce God to a simple set of rules that can be manipulated by anyone to make clear predictions, like we can in theories of modern physics? If not, God will always remain as a theoretical option of last resort — something to be invoked only after we are absolutely convinced that no possible naturalist option can explain the universe we see.

————

Obviously these very simple points don’t come anywhere near addressing all the possible issues in this area. In particular, I haven’t made any real attempt to argue that a purely naturalistic explanation actually is a better fit to the observed universe than God or similar ideas. Instead I’ve just tried to explain the mindset of someone like me who does end up coming to that conclusion. In my paper I’ve tried to lay out why invoking God doesn’t seem to provide an especially promising explanation of the world around us. Others may disagree, but I hope this has made things more clear.

I’m a big believer that academic disciplines should engage in messy interactions, not keep demurely separate from each other. But it’s a tricky business. Just because I’m (purportedly) an expert in one thing doesn’t make me an expert in everything else; on the other hand, it is possible that one area has something to offer another one. So I am in favor of dabbling, but with humility. It’s good for people to have thoughts and opinions about issues outside their immediate expertise, and to offer them in good faith, but it’s bad if they become convinced that experts in other areas are all idiots. So when you find yourself disagreeing with the consensus of expertise in some well-established field, it might very well be because of your superior insight and training, or maybe you’re just missing something. Hopefully in an exchange like this I have something to offer without making too many blunders that would make real experts cringe.

Here’s a sample of the interview.

SC: I would be extremely suspicious of any attempts to judge that the world must ‘necessarily’ be some way rather than any other. I can imagine different worlds—or at least I think I can—so I don’t believe that this is the only possible world. That would also go for any particular feature of the laws this world follows, including their stability. Maybe the laws are constant through time, maybe they are not. (Maybe time is a fundamental concept, maybe it isn’t). We don’t yet know, but it seems clear to me that these are empirical questions, not a priori ones. Because we want to understand the world in terms that are as simple as possible, the idea that the underlying laws are stable is an obvious first guess, but one that must then be tested against the data. Said in a slightly different language: any metaphysical considerations concerning what qualities the world should properly have can be taken seriously and incorporated into Bayesian priors for evaluating theories, but ultimately those theories are judged against experiment. We should listen to the world, not decide ahead of time what it must be.

Dan Sperber, via Henry Farrell, suggests that we celebrate by posting quotes from Hume. When I first encountered him as a college freshman, it was in the context of a theology course where we were reading Dialogues Concerning Natural Religion. I was intrigued when our professor pointed out a passage that seemed to prefigure Darwin’s theory of natural selection, which wasn’t going to appear until 82 years later. My dog-eared copy seems to have gone missing, but I found the quote at The Rough Guide to Evolution.

“And this very consideration too, continued PHILO, which we have stumbled on in the course of the argument, suggests a new hypothesis of cosmogony, that is not absolutely absurd and improbable. Is there a system, an order, an economy of things, by which matter can preserve that perpetual agitation which seems essential to it, and yet maintain a constancy in the forms which it produces? There certainly is such an economy; for this is actually the case with the present world. The continual motion of matter, therefore, in less than infinite transpositions, must produce this economy or order; and by its very nature, that order, when once established, supports itself, for many ages, if not to eternity.

But wherever matter is so poised, arranged, and adjusted, as to continue in perpetual motion, and yet preserve a constancy in the forms, its situation must, of necessity, have all the same appearance of art and contrivance which we observe at present. All the parts of each form must have a relation to each other, and to the whole; and the whole itself must have a relation to the other parts of the universe; to the element in which the form subsists; to the materials with which it repairs its waste and decay; and to every other form which is hostile or friendly. A defect in any of these particulars destroys the form; and the matter of which it is composed is again set loose, and is thrown into irregular motions and fermentations, till it unite itself to some other regular form.”

To me now, it looks like something of a cross between Darwin — successful forms persevering among the chaos — and the Lucretius/Boltzmann scenario of the universe coming into existence through the random motion of atoms. (What makes Lucretius and Hume brilliant thinkers but Boltzmann and Darwin influential scientists is that the latter grappled closely with data, not just with ideas.)

The common thread among all these thinkers: trying to explain the origins of order in the absence of teleology. The fact that we can do that successfully in biology, and are hot on the trail in cosmology, is a milestone achievement in the history of human thought.

Carrier goes to great lengths to explain that these moral facts are not simply “out there” in the same sense that the laws of physics arguably are, but rather that they express relationships between the desires of particular humans and external reality. (The useful analogy is: “bears are scary” is a true fact if you are talking about you or me, but not if you are talking about Superman.)

I don’t buy it. Not to be tiresome, but I have to keep insisting that you can’t squeeze blood from a turnip. You can’t use logic to derive moral commandments solely from facts about the world, even if those facts include human desires. Of course, you can derive moral commandments if you sneak in some moral premise; all I’m trying to say here is that we should be upfront about what those moral premises are, and not try to hide them underneath a pile of unobjectionable-sounding statements.

As a warm-up, here is an example of logic in action:

• All men are mortal.
• Socrates is a man.
• Therefore, Socrates is mortal.

The first two statements are the premises, the last one is the conclusion. (Obviously there are logical forms other than syllogisms, but this is a good paradigmatic example.) Notice the crucial feature: all of the important terms in the conclusion (“Socrates,” “mortal”) actually appeared somewhere in the premises. That’s why you can’t derive “ought” from “is” — you can’t reach a conclusion containing the word “ought” if that word (or something equivalent) doesn’t appear in your premises.

This doesn’t stop people from trying. Carrier uses the following example (slightly, but not unfairly, paraphrased):

• Your car is running low on oil.
• If your car runs out of oil, the engine will seize up.
• You don’t want your car’s engine to seize up.
• Therefore, you ought to change the oil in your car.

At the level of everyday practical reasoning, there’s nothing wrong with this. But if we’re trying to set up a careful foundation for moral philosophy, we should be honest and admit that the logic here is obviously incomplete. There is a missing premise, which should be spelled out explicitly:

• We ought to do that which would bring about what we want.

Crucially, this is a different kind of premise than the other three in this argument; they are facts about the world that could in principle be tested experimentally, while this new one is not.

Someone might suggest that this is isn’t a premise at all, it’s simply the definition of “ought.” The problem there is that it isn’t true. You can’t claim that Wilt Chamberlain was the greatest basketball player of all time, and then defend your claim by defining “greatest basketball player of all time” to be Wilt Chamberlain. When it comes to changing your oil, you might get away with defining “ought” in this way, but when it comes to more contentious issues of moral obligation, you’re going to have to do better.

Alternatively, you’re free to say that this premise is just so obviously true that no reasonable person could possibly disagree. Perhaps so, and that’s an argument we could have. But it’s still a premise. And again, when we get to issues more contentious than keeping your engine going, it will be necessary to make those premises explicit if we want to have a productive conversation. Once our premises start distinguishing between the well-being of individuals and the well-being of groups, you will inevitably find that they begin to seem a bit less self-evident.

Observe the world all you like; you won’t get morality off the ground until you settle on some independent moral assumptions. (And don’t tell me that “science makes assumptions, too” — that’s obviously correct, but the point here is that morality requires assumptions in addition to the assumptions we need to get science off the ground.) We can have a productive conversation about what those assumptions should be once we all admit that they exist.

1. God is by definition a perfect being.
2. It is more perfect to exist than to not exist.
3. Therefore, God exists.

Now, this is a really cartoonish version of the argument — it’s not meant to be taken seriously. This kind of ontological proof is a favorite whipping-argument for atheists, just because it seems so prima facie silly. Just ask Jesus and Mo.

This kind of mockery is a little unfair (although only a little). What’s important to realize is that the ontological proof is perfectly logical — that is, the conclusions follow inevitably from the premises. It’s the premises that are a bit loopy.

It’s instructive and fun to see this in terms of formal logic, especially because the proof requires modal logic — an extension of standard logic that classifies propositions not only as “true” or “false,” but also as “necessarily true/false” and “possibly true/false.” That is, it’s a logic of hypotheticals.

So here is one formalization of the ontological argument, taken from a very nice exposition by Peter Suber. First we have to define some notation to deal with our modalities. We denote possibility and necessity via:

Just given these simple ideas, a few axioms, and a fondness for pushing around abstract symbols, we’re ready to go. Remember that “~” means “not,” a “v” means “or,” and the sideways U means “implies.” Take “p” to be the proposition “something perfect exists,” and we’re off:

There is something beautiful here, even if it’s somewhat silly as a proof for the existence of God. It’s silly in an illuminating way!

As Suber says, the argument is “valid but unsound.” He pinpoints three premises with which reasonable people might disagree: 1 (“if perfection exists, it necessarily exists”), 2 (“perfection possibly exists”), and 5 (“if something is necessarily true, then it is necessarily necessarily true”). That last one is not a typo.

For me, the crucial mistake is some mixture of 1 and 2, mostly 2. The basic problem is that our vague notion of “perfection” isn’t really coherent. Anselm assumes that perfection is possible, and that to exist necessarily is more perfect than to exist contingently. While superficially reasonable, these assumptions don’t really hold up to scrutiny. What exactly is this “perfection” whose existence and necessity we are debating? For example, is perfection blue? You might think not, since perfection doesn’t have any particular color. But aren’t colors good, and therefore the property of being colorless is an imperfection? Likewise, and somewhat more seriously, for questions about whether perfection is timeless, or unchanging, or symmetrical, and so on. Any good-sounding quality that we might be tempted to attribute to “perfection” requires the denial of some other good-sounding quality. At some point a Zen monk will come along and suggest that not existing is a higher perfection than existing.

We have an informal notion of one thing being “better” than another, and so we unthinkingly extrapolate to believe in something that is “the best,” or “perfect.” That’s about as logical as using the fact that there exist larger and larger real numbers to conclude that there must be some largest possible number. In fact the case of perfection is much worse, since there is not single ordering on the set of all possible qualities that might culminate in “perfection.” (Is perfection sweet, or savory?) The very first step in the ontological argument rests on a naive construal of ordinary language, and the chain is no stronger than its weakest link.

If I can paraphrase, the argument is something like this: “You say that morality isn’t part of science because you don’t know what a `unit of well-being’ is — it’s not something that could in principle be measured by doing an experiment. But one could just as easily say that you don’t know what a `unit of health’ is, and therefore medicine isn’t part of science. The lack of some simple measurable quantity is a simplistic attack against a sophisticated problem.”

This gets right to the point. Because, in fact, I don’t know what a “unit of health” is, which is why medicine is not — solely — part of science.

Let me explain what I mean. Obviously we use science all the time when it comes to medicine. Similarly, we should be very ready to use science when it comes to morality — it’s an indispensable part of the endeavor. But in both cases there is a crucial component that lies outside the realm of science.

Here’s how we do medicine, in a cartoonishly simplified version that is nevertheless good enough for our present purposes. First, we decide what we mean by “healthy.” Then, we use science to try to bring it about.

That first step is not science, no matter how much science might be involved in the definition. Various measurable quantities certainly belong to the realm of science — height, weight, pulse, blood pressure, lifespan, time in the 40-yard dash, etc. But what we decide to label “healthy” is irreducibly a human judgment, not an empirical measurable. Some people might think that extreme thinness is part of being healthy, while others might prefer a more robust physique. Some people might define health as the state that maximizes life expectancy, while others might put more emphasis on quality of life even at the expense of total years. It matters not a whit what people actually think, of course — even if everyone in the world agreed on what “healthy” meant, it would still be a judgment rather than an empirical measurement. If one contrarian person came up with a different definition, they wouldn’t be “right” or “wrong” in the conventional scientific sense. There is no experiment we could do to answer the question one way or another.

In the real world, we more or less agree on what constitutes health, so the non-empirical status of this choice isn’t treated as a crucially important philosophical problem. (At least, until you start reading the literature on disability studies, and you realize that what you thought was obvious maybe is not.) We agree on what health is, and we set out to achieve it, and that second part is very much science.

Morality is exactly the same way, although with somewhat less unanimity in the first step. We agree (or not) on what morality is, and once we do the process of achieving it is very much a scientific issue, in the broad-but-perfectly-valid definition of “science” as “an understanding of how the world works based on empirical data.” Once again, it doesn’t matter whether we agree or not, because that first step is a decision we human beings make, not something we measure out there in the world.

While both health and morality are human choices rather than empirically measurable quantities, they certainly aren’t random choices. Human beings aren’t blank slates; we have preferences. Most of us would prefer to live longer and be free of aches and pains; these preferences feed into how we choose to define “health.” Likewise for morality. But “we broadly agree on X” is not, and never will be, the same statement as “X is a scientific truth.” Understanding our preferences, turning vague impulses into precise statements, constructing logical frameworks based on them — that’s what the philosophy of medicine/morality is all about.

The case of morality is actually much more difficult than the case of health, because most interesting moral questions involve tradeoffs between the interests of different people, not only the state of one individual. So even if we could do experiments to establish a unique map between mental states and human well-being, we wouldn’t really be any closer to reducing morality to science. All very fun to think about, though.

• The Moral Equivalent of the Parallel Postulate
• Sam Harris Responds
• You Can’t Derive Ought From Is

See especially the third one there, where I try to be relatively careful about what I am saying. (Wouldn’t impress a philosopher by a long shot, but by scientist/blogger standards I was careful.) Upshot: concepts relevant to morality aren’t empirical ones, and can’t be tested by doing experiments. Morality depends on science (you can make moral mistakes if you don’t understand the real world), but it isn’t a subset of it. Science describes what happens, while morality passes judgments on what should and should not happen, which is simply different.

By now Harris’s book The Moral Landscape has appeared, so you can read for yourself his explanations in full. In a different world — one where I had access to a dozen or so clones of myself with fully updated mental states, willing to tackle all the projects my birth-body didn’t have time to fit in — I would read the book carefully and report back. This is not that world.

Happily, Russell Blackford has written a longish and very good review, in the Journal of Evolution and Technology. He also blogged about it, and Jerry Coyne blogged about Russell’s review. As far as I can tell, Russell and I basically agree on all the substantive points, and he’s more trained in philosophy than I am, so you’re actually doing a lot better than something one of my clones would have been able to provide. It’s an extremely generous review, always saying “I liked the book but…” where I would have said “Despite the flaws, there are some good aspects…” So you’ll find in the review plenty of lines like “Unfortunately, Harris sees it as necessary to defend a naïve metaethical position…”

Any lingering urge I may have had to jump into the debate again in a substantive way has been dissipated by Harris’s response to Blackford’s review, which appears in the form of a letter to Jerry Coyne reprinted on his blog. It seems that very little communication is taking place at this point. Coyne paraphrases Blackford as asking “How do we actually measure well being?; for that is what we must do to make moral judgments.” Seems reasonable enough to me, and echoes very closely my first point here. Harris’s response is:

This is simply not a problem for my thesis (recall my “answers in practice vs. answers in principle” argument). There is a difference between how we verify the truth of a proposition and what makes a proposition true. How many breaths did I take last Tuesday? I don’t know, and there is no way to find out. But there is a correct, numerical answer to this question (and you can bet the farm that it falls between 5 and 5 million).

This misses the point, to say the least. The problem of measuring well-being is not simply one of practice, it’s very much one of principle. I know what a breath is; I don’t know what a “unit of well-being is.” The point of these critiques is that there is no such thing as a unit of well-being that we can look inside the brain and measure. I’m pretty sure that’s a problem of principle. Of course, Russell and Jerry and I (and David Hume, and a large number of professional moral philosophers) may be wrong about this. The way to provide a counter-argument would be to say “Here is a precise and unambiguous definition of how to measure well-being, at least in principle.” That doesn’t seem to be forthcoming.

Latter Harris says this:

The case I make in the book is that morality entirely depends on the existence of conscious minds; minds are natural phenomena; and, therefore, moral truths exist (and can be determined by science in principle, if not always in practice).

Taken at face value, this implies that truths about the best TV shows or most delicious flavors of ice cream also exist. My opinion that The Wire is the best TV show of all time is a natural phenomenon — it reflects the state of certain neurons in my brain. That doesn’t imply, in any meaningful sense, that the state of my brain provides evidence that The Wire “really is” the best TV show of all time. Nor, more programmatically and importantly, does it provide unambiguous guidance concerning which new programs should be green-lit by studio executives. The real problem — how do you balance the interests of different people against each other? — is completely ignored.

At heart I think the problem is that Sam and some other atheists are really concerned about the idea that, without objective moral truths based on science, the field of morality becomes either the exclusive domain of religion, or simply collapses into nihilism. Happily for reality, that’s an extremely false dichotomy. Morality isn’t out there to be measured like some empirical property of the physical world, but that doesn’t mean it’s impossible to be moral or to speak about morality in a rational, thoughtful way. Pretending that morality is a subset of science is, in its own way, just as much an example of wishful thinking as pretending that morality is handed down by God. We have to face up to that temptation and accept the world as it is.

———————————————————–

DYSTELEOLOGICAL PHYSICALISM

The world consists of things, which obey rules. A simple idea, but not an obvious one, and it carries profound consequences.

Physicalism holds that all that really exists are physical things. Our notion of what constitutes a “physical thing” can change as our understanding of physics improves; these days our best conception of what really exists is a set of interacting quantum fields described by a wave function. What doesn’t exist, in this doctrine, is anything strictly outside the physical realm — no spirits, deities, or souls independent of bodies. It is often convenient to describe the world in other than purely physical terms, but that is a matter of practical usefulness rather than fundamental necessity.

Most modern scientists and philosophers are physicalists, but the idea is far from obvious, and it is not as widely accepted in the larger community as it could be. When someone dies, it seems apparent that something is *gone* — a spirit or soul that previously animated the body. The idea that a person is a complex chemical reaction, and that their consciousness emerges directly from the chemical interplay of the atoms of which they are made, can be a difficult one to accept. But it is the inescapable conclusion from everything science has learned about the world.

If the world is made of things, why do they act the way they do? A plausible answer to this question, elaborated by Aristotle and part of many people’s intuitive picture of how things work, is that these things want to be a certain way. they have a goal, or at least a natural state of being. Water wants to run downhill; fire wants to rise to the sky. Humans exist to be rational, or caring, or to glorify God; marriages are meant to be between a man and a woman.

This teleological, goal-driven, view of the world is reasonable on its face, but unsupported by science. When Avicenna and Galileo and others suggested that motion does not require a continuous impulse — that objects left to themselves simply keep moving without any outside help — they began the arduous process of undermining the teleological worldview. At a basic level, all any object ever does is obey rules — the laws of physics. These rules take a definite form: given the state of the object and its environment now, we can predict its state in the future. (Quantum mechanics introduces a stochastic component to the prediction, but the underlying idea remains the same.) The “reason” something happens is because it was the inevitable outcome of the state of the universe at an earlier time.

Ernst Haeckel coined the term “dysteleology” to describe the idea that the universe has no ultimate goal or purpose. His primary concern was with biological evolution, but the conception goes deeper. Google returns no hits for the phrase “dysteleological physicalism” (until now, I suppose). But it is arguably the most fundamental insight that science has given us about the ultimate nature of reality. The world consists of things, which obey rules. Everything else derives from that.

None of which is to say that life is devoid of purpose and meaning. Only that these are things we create, not things we discover out there in the fundamental architecture of the world. The world keeps happening, in accordance with its rules; it’s up to us to make sense of it.

Leibniz was quite fond of proclaiming overarching a priori principles. Perhaps the most famous/infamous is the Principle of Sufficient Reason, which states that everything that happens does so for some good reason. But there was also the Principle of Identity of Indiscernibles, which states that if two things have all the same properties, they are really the same thing. Sounds reasonable enough (although one might worry what qualifies as a “property”), but it can get you in trouble if you take it too far.

Remember that Newton believed in absolute space — a rigid three-dimensional set of points that forms the arena in which physics takes place. Leibniz, on the other hand, claimed that space should be thought of purely in terms of relations between different points, without any metaphysical baggage of “absoluteness.” (From a modern perspective, Leibniz was closer to correct, given Galilean relativity; but once we allow for spacetime curvature in general relativity, the relational view becomes less useful.)

So far, so good. The weird part, to modern ears, comes in when we consider Newtonian cosmology. In order to explain matter in the universe, Newton departed from the strict consequences of his Laws of Motion. Instead, he imagined that empty space existed for an infinite period of time, before eventually God decided to create matter in it.

That’s the part Leibniz couldn’t go along with. He didn’t believe God would work that way, for reasons that amount to what we would now call the translational invariance of space. If God is going to create all this matter in empty space, Leibniz reasons, He has to put it somewhere. But where? Every point is equally good! Therefore there can’t be any “sufficient reason” to create it in one place rather than in some other place. Therefore there must be a deep metaphysical flaw at the heart of Newton’s theory. Interestingly, he didn’t go for “matter has been around forever,” but instead came down on the side of “there is no such thing as absolute space.”

Maybe he was worried about Boltzmann Brains?

Here are the slides from my own talk, which was supposed to be about time but ended up being more about space. Not much in the way of original research, just some ruminations on what is and is not “fundamental” about spacetime (with the caveat that this might not be a sensible question to ask). I made two basic points, which happily blended into each other: first, that the distinction between “position” (space) and “momentum” is not a fundamental aspect of classical mechanics or quantum mechanics, but instead reflects the particular Hamiltonian of our world; and second that holography implies that space is emergent, but in a very subtle and non-local way. This latter point is one reason why many of us are skeptical of approaches like loop quantum gravity, causal set theory, or dynamical triangulations; these all start by assuming that there are independent degrees of freedom at each spacetime point, and quantum gravity doesn’t seem to work that way.

Sadly the slides aren’t likely to be very comprehensible. There’s a lot of math, and the equations don’t come out completely clearly — my first time using Slideshare, so perhaps they would look better if I uploaded a pdf file rather than PowerPoint. (Hint: the slides are much more clear if you click “Menu” at the bottom left, and switch to full-screen mode.) Also I didn’t make any attempt to have the slides stand by themselves without the accompanying words. But at least this will serve as documentation that I really did give a talk at the conference, no just hang out in restaurants in Montreal.

Here’s a Philosophy TV dialogue between John Dupré (left) and Alex Rosenberg (right). They are both physicalists — the believe that the world is described by material things (or fermions and bosons, if you want to be more specific) and nothing else. But Dupré is an anti-reductionist, which is apparently the majority view among philosophers these days. Rosenberg holds out for reductionism, and seems to me to do a pretty good job at it.

John and Alex from Philosophy TV on Vimeo.

To be honest, even though this was an interesting conversation and I can’t help but be drawn into very similar discussions, I always come away thinking this is the most boring argument in all of philosophy of science. Try as I may, I can’t come up with a non-straw-man version of what it is the anti-reductionists are actually objecting to. You could object to the claim that “the best way to understand complex systems is to analyze their component parts, ignoring higher-level structures” but only if you can find someone who actually makes that claim. You can learn something about a biological organism by studying its genome, but nobody sensible thinks that’s the only way to study it, and nobody thinks that the right approach is to break a giraffe down to quarks and leptons and start cranking out the Feynman diagrams. (If such people can be identified, I’d happily join in the condemnations.)

A sensible reductionist perspective would be something like “objects are completely defined by the states of their components.” The dialogue uses elephants as examples of complex objects, so Rosenberg imagines that we know the state (position and momentum etc.) of every single particle in an elephant. Now we consider another collection of particles, far away, in exactly the same state as the ones in the elephant. Is there any sense in which that new collection is not precisely the same kind of elephant as the original?

Dupré doesn’t give a very convincing answer, except to suggest that you would also need to know the conditions of the environment in which the elephant found itself, to know how it would react. That’s fine, just give the states of all the particles making up the environment. I’m not sure why this is really an objection.

This is purely a philosophical stance, of course; it means next to nothing for practical questions. Nor does the word “fundamental” act in this context as a synonym for
“important” or “interesting.” If I want to describe an elephant, the last thing I would imagine doing is listing the positions and momenta of all its atoms. But it’s worth getting the philosophy right. I could imagine hypothetical worlds in which reductionism failed — worlds where different substances were simply different, rather than being different combinations of the same underlying particles. It’s just not our world.

This is an old question, which has come up again in a discussion that includes Russell Blackford, Jerry Coyne, John Pieret, and Massimo Pigliucci. (There is some actual discussion in between the name-calling.) Part of the impetus for the discussion is this new paper by Maarten Boudry, Stefaan Blancke, Johan Braeckman for Foundations of Science.

There are two issues standing in the way of a utopian ideal of universal agreement: what we mean by “supernatural,” and how science works. (Are you surprised?)

There is no one perfect definition of “supernatural,” but it’s at least worth trying to define it before passing judgment. Here’s Chris Schoen, commenting on Boudry et. al:

Nowhere do the authors of the paper define just what supernaturalism is supposed to mean. The word is commonly used to indicate that which is not subject to “natural” law, that which is intrinsically concealed from our view, which is not orderly and regular, or otherwise not amenable to observation and quantification.

Very sympathetic to the first sentence. But the second one makes matters worse rather than better. It’s a list of four things: a) not subject to natural law, b) intrinsically concealed from our view, c) not orderly and regular, and d) not amenable to observation and quantification. These are very different things, and it’s far from clear that the best starting point is to group them together. In particular, b) and d) point to the difficulty in observing the supernatural, while a) and c) point to its lawless character. These properties seem quite independent to me.

Rather that declare once and for all what the best definition of “supernatural” is, we can try to distinguish between at least three possibilities:

1. The silent: things that have absolutely no effect on anything that happens in the world.
2. The hidden: things that affect the world only indirectly, without being immediately observable themselves.
3. The lawless: things that affect the world in ways that are observable (directly or otherwise), but not subject to the regularities of natural law.

There may be some difficulty involved in figuring out which category something fits, but once we’ve done so it shouldn’t be so hard to agree on how to deal with it. If something is in the first category, having absolutely no effect on anything that happens in the world, I would suggest that the right strategy is simply to ignore it. Concepts like that are not scientifically meaningful. But they’re not really meaningful on any other level, either. To say that something has absolutely no effect on how the world works is an extremely strong characterization, one that removes the concept from the realm of interestingness. But there aren’t many such concepts. Say you believe in an omnipotent and perfect God, one whose perfection involves being timeless and not intervening in the world. Do you also think that there could be a universe exactly like ours, except that this God does not exist? If so, I can’t see any way in which the idea is meaningful. But if not, then your idea of God does affect the world — it allows it to exist. In that case, it’s really in the next category.

That would be things that affect the world, but only indirectly. This is where the dark matter comparison comes in, which I don’t think is especially helpful. Here’s Schoen again:

We presume that dark matter –if it exists–is lawful and not in the least bit capricious. In other words, it is–if it exists–a “natural” phenomena. But we can presently make absolutely no statements about it whatsoever, except through the effect it (putatively) has on ordinary matter. Whatever it is made of, and however it interacts with the rest of the material world is purely speculative, an untestable hypothesis (given our present knowledge). Our failure to confirm it with science is not unnerving.

I would have thought that this line of reasoning supports the contention that unobservable things do fall unproblematically within the purview of science, but Chris seems to be concluding the opposite, unless I’m misunderstanding. There’s no question that dark matter is part of science. It’s a hypothetical substance that obeys rules, from which we can make predictions that can be tested, and so on. Something doesn’t have to be directly observable to be part of science — it only has to have definite and testable implications for things that are observable. (Quarks are just the most obvious example.) Dark matter is unambiguously amenable to scientific investigation, and if some purportedly supernatural concept has similar implications for observations we do make, it would be subject to science just as well.

It’s the final category, things that don’t obey natural laws, where we really have to think carefully about how science works. Let’s imagine that there really were some sort of miraculous component to existence, some influence that directly affected the world we observe without being subject to rigid laws of behavior. How would science deal with that?

The right way to answer this question is to ask how actual scientists would deal with that, rather than decide ahead of time what is and is not “science” and then apply this definition to some new phenomenon. If life on Earth included regular visits from angels, or miraculous cures as the result of prayer, scientists would certainly try to understand it using the best ideas they could come up with. To be sure, their initial ideas would involve perfectly “natural” explanations of the traditional scientific type. And if the examples of purported supernatural activity were sufficiently rare and poorly documented (as they are in the real world), the scientists would provisionally conclude that there was insufficient reason to abandon the laws of nature. What we think of as lawful, “natural” explanations are certainly simpler — they involve fewer metaphysical categories, and better-behaved ones at that — and correspondingly preferred, all things being equal, to supernatural ones.

But that doesn’t mean that the evidence could never, in principle, be sufficient to overcome this preference. Theory choice in science is typically a matter of competing comprehensive pictures, not dealing with phenomena on a case-by-case basis. There is a presumption in favor of simple explanation; but there is also a presumption in favor of fitting the data. In the real world, there is data favoring the claim that Jesus rose from the dead: it takes the form of the written descriptions in the New Testament. Most scientists judge that this data is simply unreliable or mistaken, because it’s easier to imagine that non-eyewitness-testimony in two-thousand-year-old documents is inaccurate that to imagine that there was a dramatic violation of the laws of physics and biology. But if this kind of thing happened all the time, the situation would be dramatically different; the burden on the “unreliable data” explanation would become harder and harder to bear, until the preference would be in favor of a theory where people really did rise from the dead.

There is a perfectly good question of whether science could ever conclude that the best explanation was one that involved fundamentally lawless behavior. The data in favor of such a conclusion would have to be extremely compelling, for the reasons previously stated, but I don’t see why it couldn’t happen. Science is very pragmatic, as the origin of quantum mechanics vividly demonstrates. Over the course of a couple decades, physicists (as a community) were willing to give up on extremely cherished ideas of the clockwork predictability inherent in the Newtonian universe, and agree on the probabilistic nature of quantum mechanics. That’s what fit the data. Similarly, if the best explanation scientists could come up with for some set of observations necessarily involved a lawless supernatural component, that’s what they would do. There would inevitably be some latter-day curmudgeonly Einstein figure who refused to believe that God ignored the rules of his own game of dice, but the debate would hinge on what provided the best explanation, not a priori claims about what is and is not science.

One might offer the objection that, in this view of science, we might end up getting things wrong. What if there truly are lawless supernatural actions in the world, but they appear only very rarely? In that case science would conclude (as it does) that they’re most likely not supernatural at all, but simply examples of unreliable data. How can we guard against that error?

We can’t, with complete confidence. There are many ways we could be wrong — we could be being taunted by a powerful and mischievous demon, or we and our memories could have randomly fluctuated into existence from thermal equilibrium, etc. Science tries to come up with the best explanations based on things we observe, and that strategy has great empirical success, but it’s not absolutely guaranteed. It’s just the best we can do.