Downward Causation

By Sean Carroll | August 1, 2011 11:03 am

Reading about emergence and reductionism and free will and determinism has led me to finally confront a concept I had vaguely heard about but never really looked into before: downward causation, a term that came to prominence in the 1970’s. (Some other views: here, here, here.) I think it’s a misguided/unhelpful notion, but this is way outside my area and I’m happy to admit that I might be missing something.

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.

CATEGORIZED UNDER: Philosophy, Science, Top Posts
  • JW Mason

    It seems like you’re simply on the anti-reductionist or emergentist side here. You and Fodor agree that talking about human beings in terms of biological or chemical processes is not necessarily preferable than talking in terms of mental states, and that in some cases there is no (useful) way of talking about mental states in terms of a “lower” or more disaggregated level at all. Where’s the disagreement?

  • Sean

    [This was a response to JW’s original comment, which was completely different from the edited comment above — hopefully it still makes sense.] Depends on what you mean by “independent.” What you are pointing out is that there are many different kinds of atomic-level configurations that would reasonably be called “this post,” whether displayed on a different computer or printed out. But whenever those configurations exist, they always correspond to “this post.” And there are other configurations (random piles of dirt, or the Magna Carta) which will never correspond to this post. So they’re not really “independent.”

    It is of course possible to talk about properties of this post independently from talking about any of its physical manifestations, but that’s completely compatible with what I am saying. (Indeed it is arguably the point.)

  • Jason Dick

    “Knowing that you’re made of elementary particles obeying the laws of physics doesn’t make you any less of a person.”

    Exactly! Just like knowing that a house is made of some combination of wood, stone, metal, glass, plastic, and other assorted things doesn’t make it any less of a house.

  • Aaron

    JW Mason: They’re both disagreeing with a straw-man version of reductionism. Only one is taking the strawman version seriously; the other one recognizes it a creation of the Emergentists.

  • Nullius in Verba

    Is the second law of thermodynamics the result of the microscopic physics of atoms and quanta (e.g. quantum irreversibility), or does it come from a higher-level boundary condition, that the past boundary of the region of study tends to have lower entropy? In this case, is the microscopic explanation the more complete?

  • Sean

    JW– For one thing, I don’t think there is any such thing as “downward causation.” For another, I am completely happy with “deriving” what happens at one level from what happens at another. I don’t think this would make me a very good Emergentist. But if it does, then I am happy that we finally all agree.

  • David Santo Pietro

    I have to admit that this feels like a cop out. We either have free will or we don’t. To say that free will is in some sense “real”, and at the same time admit that the evolution of physical particles allows no deviation sounds wishy washy.

    Baseball and free will don’t seem like very good analogies. Baseball exists when a collection of particles is arranged into a macroscopic state (people, rawhide, grass) which evolves according to what we classify as a baseball game. You can’t just point to a collection of particles and say that you saw free will. There are either multiple outcomes and actions available to us or there is only one. The fact that we can have the illusion of free will, if we don’t think about it too deeply is not really that comforting.

    Also, don’t we “cross the stream” all the time (in the reverse direction) when we claim that knowing information about protons and neutrons gives us enough knowledge to say where our feelings/sensations come from and what they are?

  • Arun

    ““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.”

    “I choose to move my hand” and the hand did not move could be because a nerve was severed or could be because the experimenter suddenly put my hand in a vise grip. I don’t see how you can avoid mixing levels; nor how you can provide a concise description of a phenomenon without mixing levels. (e.g., do I need to describe the experimenter down to her neurons and other cells?).

  • spyder

    The preeminent anthropologist Jacques Maquet responded, to a seminar paper i presented, by walking up to the chalkboard and drawing an expanding spiral. He then turned and said: “No, it is not about a closed circle, nor about straight lines.”

  • NickM

    Don’t overlook Downward Entailment. A tad more subtle. Google: russ abbott downward entailment.

    As far as it relates to philosophy of mind you might define human intelligence, even mind and consciousness, as a set of higher-level functions (epiphenomena kinda) emerging from the (condensed matter, in this case) physics of the brain. (Abbott doesn’t say this; it’s my own extrapolation. His paradigm is the Game of Life.)

    The relationship seems somehow interactive: the high-level functions are dependent on the physics which they nevertheless emerge to constrain. So how come this isn’t the same thing as plain old Downward Causation? Apparently it just isn’t. My mind is still working to wrap itself around this stuff.

  • Sam Gralla

    This would be all well and good if anybody had demonstrated free will as an emergent phenomenon. As it is, the existence of free will in the higher-level description presents a hint of new physics in the lower-level description.

  • Katie

    I’m with you until the last couple of paragraphs. Multilevel explanations are the bread and butter of the sciences (granted, some more than others). So charming references to Ghostbusters aside, it seems like the really interesting issues around reduction (at least for people happy to walk away from banal ontological arguments based on intuition and positivist fancies) are around how different levels of explanation can be integrated. You suggest rather breezily that something like Nagelian bridge principles could be used for derivation, but don’t make clear how doing so differs from crossing the streams.

  • SpeakerToManagers

    Whether we think of different organizational levels as layered vertically or side-by-side, there are still situations in which effects in different levels get entangled. For instance, long-distance correlations in systems can result in changes in microscopic structure, even while changes in microscopic states create the long-distance correlations. This isn’t anything spooky or non-physical, just the result of entangled causal effects between layers. Its probably a lot more common in the more complex systems we find in biology than in physical systems, but I think that’s because it’s very common in systems containing feedback loops, which are common in living organisms, even the simplest ones.

    It’s much easier to talk about this sort of thing in the context of medium-scale systems like living cells than when talking about human nervous systems and mental states, especially because we have special prejudices regarding our own mental functions. That’s why I think discussions of the nature of system organization and its relationship to reductionism tends to get derailed when we try to talk about human consciousness and psychology. Although the same discussion as it relates to animal development and the nature of genetics ran through a lot of acrimonious debate for most of the 20th century.

  • Sean

    I don’t want to give the impression that I think I’ve said the last word on the relationship between different levels (although I’d prefer “vocabularies” or “theories” to “levels”). Sure, they can get complicated, and the disentangling isn’t always easy.

    I think the relevant question is: does your purported vocabulary support an autonomous understanding of properties and dynamics? That is, when I talk about fluids, I don’t ever have to talk about atoms (as long as I’m in the range of validity of the fluid description). If that’s the case, I don’t need to invoke different vocabularies. If it’s not the case, then the sensible thing to do would be to add more elements to the vocabulary until you had an autonomous model. Of course it might be very very hard to actually put together a description that is both autonomous and self-consistent as well as accurate and interesting. Science is hard! But I don’t see any argument that it’s generically impossible (although that might be true — again, this is not my area).

    The more general point remains: having a Laplace’s-Demon-level of perfect microscopic understanding wouldn’t dissuade me from speaking the language of mental states and choices, and treating them as real.

  • Craig

    The problem with your exclusion principle argument is that it only applies to the physical properties of the system. It doesn’t necessarily apply to the patterns found in the system. Dennett has a nice example: Its a nice little story and I don’t want to ruin the ending, but I think it shows the limits of reductionism. Some facts about the higher level can’t be explained in terms of facts about the lower levels even in principle.

  • NickM

    Reductionism probably dies anyway at the level of strong fermionic interaction (which gives rise to the Fermion Sign Problem a.k.a. the Numerical Sign Problem, N-Body Problem, Many-Body Problem, Chiral Fermions on the Lattice and a few other noms de guerre). Fermionic interaction isn’t mathematically formalizable. Computationally it’s NP-Hard. Like protein folding, it can’t be modeled, only statistically described.

    “Determinism” and “Free Will” are probably signposts of terminal ignorance. Am I the subject choosing or the object chosen? All we know is the occurrence of a choice.

  • Craig

    Also I would point you to Douglas Hofstadter’s “I am a Strange Loop” where he argues that in order to understand minds you have to understand there paradoxical level shifting nature.

  • Craig

    Also how are we supposed to make sense of phrases like “I turned the car key”? Is the phrase just not allowed when we are being technical?

  • Tyler

    Sean, if you reject downward causation then you must also reject ‘upward causation.’ To me, this is also an argument about induction and deduction, and which approach is more valid. Put in these terms, we know that both are, yet we do have our preferences, i.e., should I build up from a particular set of empirical facts, or down from a more general principle. As we know, it is best to engage both approaches when trying to solve a problem.

    Your statement: “what effect do my feelings have on my protons and neutrons?” is like going from a pyramid’s tip to a grain of sand at its base in one leap. Though ‘extreme,’ doing so is certainly not impossible. For example, if I feel crummy I don’t exercise which increases the count in my body of a particular configuration of protons and neutrons in the form of cholesterol. Thus feeling crummy equals a specific increase in the number of my protons and neutrons, and feeling good decreases them. This is of course crude because in addition to this there are many other factors altering the configuration count of my protons and neutrons. Thus the statement, taken as an extreme example of downward causation, reveals that the relation between the items is simply more crude, but never meaningless.

    Reversing the statement in order to put it in terms of upward causation, it reads: “what effect do my protons and neutrons have on my feelings?” And from this, though again crude, we know that when a particular configuration of my protons and neutrons decrease I must also be happy.

  • Baby Bones

    I think that the job of reductionism to account for novel macroscopic phenomena begins at the microscopic level and can work its way to conceptually incommensurate but nonetheless effective bigger pictures. Reductionism works well in describing how many strongly interacting systems, e.g., electrons in a metal, can behave as many non-interacting systems, e.g., a free electron gas, or be described accurately in an even more abstract way as a two-body system, e.g., an electron above the Fermi level, a hole below the Fermi level. It can accurately build up a picture wherein holes are imagined to be particles with a mass. Now these new entities effectively describe measurable quantities but are fictitious. We know that a hole is not actually a particle but it is quite valid to talk about a hole gas, just like it is to talk about the closer-to-reality electron gas.

    The thing is though, there’s lots of stuff the microscopic picture will never be able to deal with, for it involves too much complexity, whereas these bigger, yet somehow artificial, pictures can deal with the complexity. I do not think that this ability is entirely due to our ingenuity at making diagrams to account for stuff. In fact, these bigger pictures are better at explaining the nature of the phenomena than the microscopic pictures ever will be. A first-principles calculation is always going to be an vast accounting ledger that provides little insight other than to explain a mysterious aspect to a bigger picture or to confirm the validity of the bigger picture in a simple case.

    As this is a general state of affairs in science, I suppose there is a general principle at work and i will say it is called “decoupling microscopic pictures from macroscopic pictures”, and there is reality to both pictures.

    I guess essence for me is the opposite of its normal definition. To me, it is what effectively happens, not what lies underneath; in other words, the essence of the matter is not the essence of matter. Hence, people are free agents, and statements to that like can be formulated free of atomic considerations once you can eliminate certain couplings from consideration (usually, they mostly cancel out).

    In my opinion, psychology can be completely decoupled from physics (one day, scientists will renormalize psychology), but we have yet to find a way do so with consistency, and it is as yet a fruitful an endeavor to consider the physics of neurons and neural networks.

  • Bee

    Your argument about the # of equations and unknowns becomes a little wobbly if you have an infinite amount of both. What’s the number of constituents of a baseball? I’m also not sure the distinction between micro- and macroscopic as being a matter of size is waterproof. Anyway, that’s not to say that I disagree with you.

  • Scott Aaronson

    Sean, I sometimes fantasize that the way to dissolve all debates about reductionism vs. holism would be to teach everyone computer programming! But then I remember that that can’t be the whole solution, because programmers get into these debates too.

    Seriously, while there are lots of really terrible “anti-reductionist” claims — some of which you demolished in this post and previous ones! — there’s ONE “anti-reductionist” claim that I’ve been able to make sense out of. This is that, not only do there exist “effective” higher-level laws that everyone uses in practice, rather than rederiving everything from the lower-level laws — an obvious point that any reductionist would agree with — but many of the higher-level laws would very plausibly be the same, even if the lower-level laws had been different.

    So for example, we could easily imagine Darwinian natural selection operating in a universe with completely different laws of physics than ours. But if so, then in that specific sense, we could think of natural selection as enjoying a sort of “autonomy” from lower-level physics. By analogy, if I run a Java applet on my PC, it doesn’t seem right to say that the “true, lowest-level description” of the applet resides in the machine-code instructions that my PC actually executes, the Java bytecode being merely a “higher-level description” of what’s going on — since exactly the same Java bytecode could also run on (say) Mac or Linux, in which case it would correspond to completely different machine code.

    (Saul Kripke made a similar point in his famous book Naming and Necessity, where he asked questions like: “would water still be water if it had turned out to be something other than H2O?”)

  • Phillip Helbig

    A good, no-nonsense description of various levels, their relationships among one another, emergence etc (among many other things) is to be found in Robert Pirsig’s book Lila. I recommend reading his first book beforehand, namely Zen and the Art of Motorcycle Maintenance. (As Pirsig himself says, don’t take the title too literally. In the same spirit as acronyms for physics experiments, having a nice ring to it is more important than having it be a good description of the thing it titles.) Even if one doesn’t agree with everything in his books, a) they are still a good read and b) they are a good summary of various aspects of science and philosophy which are not found together in many other places.

  • Phillip Helbig

    @22: Good point. Pirsig gives a similar example in one of his books: Give someone with a knowledge of electronics all the tools he wants and a hard disk. Task: Find the novel residing in a text-processing system on the hard disk. While in some sense the novel is, of course, there and could, in principle, be found (though in practice this would probably require a knowledge of the operating system, the text-processing program and the hard disk, in addition to a general knowledge of electronics), most would agree that that is merely one possible representation of the “ideal” novel (I hear my thoughts echoing inside of Plato’s cave now) and hence that the description of the novel per se is, in this case, more fundamental than its representation.

    Of course, Abstruse Goose hits the nail on the head with regard to the relationships between levels: .

  • Phillip Helbig

    “So for example, we could easily imagine Darwinian natural selection operating in a universe with completely different laws of physics than ours.”

    An interesting spin on this is James P. Hogan’s book Code of the Life-Maker; highly recommended. (For the record: I grew up on Asimov (whom I met personally once) and Clarke and could never really get into most other s.f. (I was not impressed by, say, Heinlein’s Stranger in a Strange Land (though it is a great title). Some of Hogan’s books are quite good, some are OK. There are probably some which I haven’t read which aren’t good at all. (Of course, we all know that Wagner’s music is better than it sounds and one should, at least in some circumstances, distinguish between the artist and the art. In particular, anyone who finds Hogan rather unpalatable as a result of relatively recent public statements can rest assured that there is no hint of this in his books, at least in the earlier ones (which I have read).) I am not ruling out the fact, though, that there might be other s.f. writers, young or old, who are worth reading for me.)

  • Igor Khavkine

    @Scott, sounds like you’ve just described the subtle difference between isomorphism and equality. I can only see how making that point being anti-reductionist if reductionism claimed that non-equality implies non-isomorphism, but I doubt many reductionists take such a claim seriously.

    @Bee, do you mean that Sean’s argument about the number of equations and unknowns is wobbly or only appears wobbly? I certainly appreciate that it could appear wobbly to the semi-initiated, those familiar enough with infinities to see that there could be a problem but not sufficiently familiar the fact that well-posedness can be still be established in many infinite dimensional systems of equations relevant to physics, once reasonable constraints are imposed on potential solutions (continuity, etc.).

  • Igor Khavkine

    @Phillip#24 and @NickM#15, these are both very difficult examples of code breaking. I think the difficulty there (which I think is unarguable) is much more tied to the notion of what it means to break a code, rather than reductionism per se.

  • Rationalist

    Scott Aaronson > “we could think of natural selection as enjoying a sort of “autonomy” from lower-level physics”

    You may be confusing two different kinds of “autonomy”; evolution is “autonomous” from physics in a broad sense: if you have a multiverse of universes, each with different physics, many of them will have evolution. But evolution is not autonomous from physics in detail: in any specific instance of evolution, if evolutionary theory predicts X and physics predicts Y, Y will happen. (For example, an otherwise highly fit genotype could by pure chance cause the DNA molecule which encodes it to become unstable, so fitness would fail to increase as you might predict if your theory didn’t talk about DNA)

  • Ryan

    As stated by Craig, this conundrum is right in Hofstadter’s wheelhouse.

    Hofstadter’s proposition takes your parallel equivalence a step further. He also names the trade off, although not as rigorously as some would like: Moving “Up” hierarchical levels sacrifices some informational resolution, but increases comprehensibility. He, like you, contends that descriptions at either the level of physics or the level of percepts. I really like that you noted that the “higher level” must be a fully autonomous causal system – another way of framing this self-consistency may be to say that the noise in the lower levels is insufficient to affect causal relationships at the higher level.

    What’s particularly interesting to me is the relationship between levels of physical reality and “conceptual reality”, if we are to take the mind as a real thing. Hofstadter has a lot to say about this in terms of the emergence from “level crossing” feedback loops, and draws an analogy to the Godel incompleteness theorems, which is sort of a favorite stone to throw at reductionists (Penrose hurls it, as well, in “The Emperor’s New Mind”).

    Myself, I don’t think such arguments about level crossing are all that interesting. What is interesting is the structure of information flows at the physical level, and how it relates to the information geometry of the mind. These types of correlations are what are ultimately useful to those with the engineering ambition to build (and therefore understand) minds that include the features of the human mind. The computer example is a very good one, and I’d recommend Charles Petzold’s “Code” for anyone wanting to explore that particular example in excruciating detail.

  • Scott Aaronson

    Rationalist #28:

    You may be confusing two different kinds of “autonomy”

    I was careful to say a “sort of” autonomy, since I know there’s also an obvious sense in which evolution can’t be autonomous! (Namely, once you’ve fixed all the facts about the laws of physics and the initial/boundary conditions, you’ve also fixed the observable higher-level behavior, at least probabilistically. That’s true essentially by definition: if there were some other determinant of the higher-level behavior, we’d simply add it to our description of physics!)

    in any specific instance of evolution, if evolutionary theory predicts X and physics predicts Y, Y will happen

    Not so fast: if our theories of evolution and physics were both “perfect”, then clearly their predictions would never disagree! (In particular, a “perfect evolutionary theory” would need to take as input every possible contribution to fitness, which certainly includes DNA-level contributions.)

    If, on the other hand, both theories were imperfect, then it’s of course possible that the evolutionist would predict X, the physicist would predict Y, and X would be right (the usual example being Lord Kelvin’s despite with biologists about the age of the sun). So maybe what you really mean is that we can imagine a “perfect” theory of fundamental physics, but not a similarly “perfect” theory of evolution?

  • Mike

    No high level phenomena is independent of physics — nothing is independent of physics. The point, however, is that really being able to “explain” some high level phenomena generally is only possible by reference to other high level phenomena. Of course, these other high level phenomena too are not independent of physics. And the physics underlying each phenomena is related — physically, not just metaphorically — but almost always in some very attenuated way so that, as far as we know, even if we had a complete description of initial conditions, the actual physical relationship can never be computed, if only because such computation is practically intractable. Moreover, assuming that it could be computed, what exactly would such a “physical” description tell us about the high level phenomena?

    My favorite example of this come from David Deutsch:

    “There are explanations at every level of hierarchy. Many of them are autonomous, referring only to concepts at that particular level (for instance, ‘the bear ate the honey because it was hungry’). Many involve deductions in the opposite direction to that of reductive explanation. That is, they explain things not by analyzing them into smaller, simpler things but by regarding them as components of larger, more complex things– about which we nevertheless have explanatory theories. For example, consider one particular copper atom at the tip of the nose of the statue of Sir Winston Churchill that stands in Parliament Square in London. Let me try to explain why that copper atom is there. It is because Churchill served as prime minister in the House of Commons nearby; and because his ideas and leadership contributed to the Allied victory in the Second World War; and because it is customary to honor such people by putting up statues of them;
    and because bronze, a traditional material for such statues, contains copper, and so on. Thus we explain a low-level physical observation– the presence of a copper atom at a particular location– through extremely high-level theories about emergent phenomena such as ideas, leadership, war and tradition.

    Presumably a reductive ‘theory of everything’ would in principle make a low-level prediction of the probability that such a statue will exist, given the condition of (say) the solar system at some earlier date. It would also in principle describe how the statue probably got there. But such descriptions and predictions (wildly infeasible, of course) would explain nothing. They would merely describe the trajectory that each copper atom followed from the copper mine, through the smelter and the sculptor’s studio, and so on. They could also state how those trajectories were influenced by forces exerted on surrounding atoms, such as those compromising the miners’ and the sculptor’s bodies, and so predict the existence and shape of the statue. In fact such a prediction would have to refer to atoms all over the planet, engaged in the complex motion we call the Second World War, among other things. But even if you had the superhuman capacity to follow such lengthy predictions of the copper atom’s being there, you would still not be able to say, ‘Ah yes, now I understand why it is there.’ You would merely know that its arrival there in that way was inevitable (or likely, or whatever), given all the atoms’ initial configurations and the laws of physics. If you wanted to understand why, you would still have no option but to take a
    further step. You would have to inquire into what it is about that configuration of atoms, and those trajectories, that gave them the propensity to deposit a copper atom at this location. Pursuing this inquiry would be a creative task, as discovering new explanations always is. You would have to discover that certain atomic configurations support emergent phenomena such as leadership and war, which are related to one another by high-level explanatory theories. Only when you knew those theories could you understand fully why that copper atom is where it is.”

  • NickM

    One thing the lower-level physics and the higher-level “epiphysics” have in common is that they’re carriers of information. Information can be thought of as analogous to energy (it’s not mandatory to think of it that way, but it can be done). Like energy, information is a hypothetical construct knowable only through its manifestations. Like energy, information comes coded and needs a transponder in order to be communicated. (You can’t turn on an air-conditioner that runs on electrical energy by dropping it from an airplane and expecting it to operate on kinetic energy. You can’t yell at me in Nahuatl to watch out for the open manhole I’m about to step into and expect me to heed your warning. There’s no point in screaming at your laptop unless it’s loaded with the appropriate voice-recognition software and is turned on.)

    The nexus between brain-physics and high-level functions is an informational transpondence. To understand how it works you need to know all the codes involved. Talk to the fermion.

  • Tom Clark

    Sean, thanks for a nice excursus on the validity of different levels of explanation – they can peacefully coexist without one claiming ontological priority as more real than the others.

    “It’s only when you start asking ‘what effect do my feelings have on my protons and neutrons?’ that you start getting syntax errors.”

    Quite right. And it applies at the level of the neural correlates of feelings (a subset of conscious phenomenal experience) as well: there’s no accepted theory about how the correlates entail feelings, or how feelings could have an effect on their correlates. And it’s difficult to identify one with the other, since after all the correlates are in the public domain, feelings in the private (no one has ever observed a pain or any other conscious experience in the way we observe their neural correlates). So when you say “You are allowed to treat mental phenomena (or whatever) as perfectly ‘real’ and causally efficacious,” it might still be a mistake to suppose that phenomenal experience can play a causal role in neuroscientific or otherwise physicalist explanations of behavior. Rather you’ve got two parallel explanatory tracks, one physicalist and one experiential, and they are mutually irreducible even though highly correlated, This solves, or perhaps dissolves, the problem of mental causation when it comes to things like feelings.

    “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.”

    Different, yes, but regarding the above: The phenomenal and physical aren’t different descriptions of a single underlying reality, but (I speculate) *constitute* two parallel “realities” generated by two epistemic perspectives, a subjective phenomenal reality of qualitative conscious states entailed (somehow – the hard problem of consciousness) by being an individual cognizer modeling the world, and a physical reality of non-qualitative concepts and theories generated by the intersubjective project of science modeling the world,

  • Sean

    Scott, I do agree with the claim that many different lower-level laws can give rise to the same higher-level laws. But I’m not sure what lesson — good or bad — you’re trying to draw from it.

    In particle physics, this feature is not only true but obvious and occasionally frustrating. It’s a consequence of the renormalization group; no matter what happens at high energies, low energy physics can be essentially unchanged. (We say that high-energy particles “decouple” from the low-energy phenomena.) That makes us sad, because we can’t simply do arbitrarily precise experiments at low energies to discover new massive particles; we have to build giant particle accelerators that actually reach high energies, and that costs money.

    There is a slightly different flavor to things when you move to more slippery realms like computer programs or for that matter texts like blog posts. (I was addressing this a bit in comment #2.) It’s certainly true that very different substrates can represent the same higher-level processes. I think that’s an extremely interesting feature of the world, but don’t necessarily see that it counts as an “anti-reductionist” argument.

  • AnotherSean

    Reductionism is the most powerful tool in the history of science, and it seems like almost every “emergent” victory contains some implicit reduction. Still, I could see how it comes to an end.
    The Godel proof shows how in mathematics. In physics it may turn out that particle theory is determined by geometry. I don’t know if these examples vindicate holism, per se, but they don’t seem to be examples of classic reductionism either. Probably, in the end, the debate becomes meaningless.

  • Scott Aaronson

    Sean: I don’t think the sensible type of reductionism that you defended in this post (and that I also believe) is challenged in the slightest by “decoupling” phenomena—sorry if I wasn’t clear about that!

    Since (like essentially all reductionism-debates among sensible people :-) ) this one just boils down to the meanings of words, let’s define “super-reductionism” to be the following theory:

    Higher-level phenomena ARE just aggregates of lower-level phenomena. In other words, not only are they causally determined by lower-level phenomena, but they can’t even be properly defined without reference to the lower levels.

    My point was just that, if someone was tempted to believe super-reductionism—or if they thought that super-reductionism was the inevitable consequence of ordinary reductionism—then they could be cured by thinking about decoupling phenomena. Of course, this fits in perfectly well with what you said yourself:

    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.

    I’d go maybe just a hair further: not only is a baseball game as real as the subatomic particles involved in it, but if we “swapped out” the subatomic particles and replaced them by a different physical substrate, it would still be the same game!

  • John Merryman

    I think prime numbers are a good example of emergence. As such they are the intersection of two different numeric ordering systems; division/multiplication and sequential addition. Both are mathematically foundational, but not identical. What this suggests is there many not be a singular bottom level.
    Yes, there are smaller levels, subatomic particles, etc, but does size equate to foundational? What if there is no Higgs? Could the effect of mass be generated by field effect of some equilibrium state? What if there is some spacetime geometry affecting the motions of the macrocosm? Wouldn’t it be as foundational as subatomic quantum behavior? If the entire universe originates/exists in a singularity, wouldn’t it be macrocosmically foundational and potentially exert a downward influence equal to the bottom up influence of QM?
    The laws governing outcomes may well be deterministic(or they wouldn’t be laws), but is the input a given?

  • Rationalist

    @Scott Aaronson: You know, you have got me thinking now. Could we have such a thing as a “perfect” high-level theory?

    Initially I thought not, but actually I think there are such things.

    Any logical tautology with a well-defined mapping from predicates (etc) onto properties of physical reality would count.

    To the extent that high-level theories are imperfect, the imperfection hides in the details of the mapping.

    So you could make a perfect high-level-theory by choosing an artificially simple mapping, e.g. if you have five objects and then create another three, you have eight. For suitably strict definitions of “object”, you now have a perfect high-level theory.

    Does this help a believer in causally efficacious free will? Perhaps if you are prepared to say that a law of mathematics or computing causes effects in any physical system which satisfies its axioms… …you could argue that there is some program which constitutes your decision algorithm with the property that your brain is an instantiation of it. But I don’t think that the free will supporter would really want to think of herself as a program any more than she would want to think of herself as a collection of atoms. Certainly, it is an interesting train of thought, and from my reading of Dennett, Gary Drescher, Drew McDerrmot, etc it’s basically the route that some modern determinists/reductionists go down anyway. And I guess it answers the original question: is there downward causation? Well to the extent that you think that mathematics can cause things, there is.
    Personally I like my cause and effect to be between concrete objects like electrons.

  • Mike

    @ Scott Aaronson,

    “I’d go maybe just a hair further: not only is a baseball game as real as the subatomic particles involved in it, but if we “swapped out” the subatomic particles and replaced them by a different physical substrate, it would still be the same game!”

    I agree with the point of your posts, but with respect to the above quote, aren’t you just assuming that a particular change to the physical substrate ultimately has no impact on the game? At least in theory, particular changes (or a complex of changes) could “change” the game, couldn’t they?

    Of course, as far as we know in most all cases it would be impossible to compute and predict what such changes would consist of, but in a sense that just reinforces the conclusion that higher level explanations do have a certain autonomy.

  • Scott Aaronson


    Could we have such a thing as a “perfect” high-level theory?

    Awesome question! As I see it, if we have a theory that describes (say) the behavior of some computer program, then not only is that theory not necessarily “approximate” by virtue of computers being complicated, composite objects; it might well be more accurate than our theory of the subatomic particles that the computer is made of! Of course a meteorite could always hit a computer and make it stop working. But crucially, most people wouldn’t call that a defect in a theory about computer programs—they’d just say that the (broken) computer is now a system to which the theory no longer applies.


    with respect to the above quote, aren’t you just assuming that a particular change to the physical substrate ultimately has no impact on the game? At least in theory, particular changes (or a complex of changes) could “change” the game, couldn’t they?

    You’re right: in this sort of Kripkean thought experiment, the ground rule is that we assume no changes to the observable, macroscopic features of the game. In principle, that might be hard to arrange, if (for example) the decision of which team went to bat first had to be made by observing the behavior of a particle accelerator! So I guess it’s important for this thought experiment that baseball has no such rule.

  • Mike

    @ Scott Aaronson,

    “In principle, that might be hard to arrange, if (for example) the decision of which team went to bat first had to be made by observing the behavior of a particle accelerator! So I guess it’s important for this thought experiment that baseball has no such rule.”

    Can’t disagree — that would be a lousy rule. I was thinking more along the lines that it sure would be hard to specify the physical substrate in such a way as to result in Abner Doubleday (or Alexander Cartwright, depending on your preference) deciding that the base paths would be an oval rather than diamond — it would cut down on infield hits and stolen bases by a large measure and it would certainly be much harder to score from first on a double. Unless, of course, those changes also led him to shorten the distance between bases by some “corresponding” length. 😉

  • NickM

    “The Game of Life rules are analogous to the fundamental laws of physics. They determine everything that happens on a Game of Life grid. Nevertheless there are higher level laws that are not derivable from them.

    “Certain Game of Life configurations create patterns. The most famous is the glider, a pattern of on and off cells that moves diagonally across the grid. It is possible to implement an arbitrary Turing machine by arranging Game of Life patterns. Computability theory applies to such Turing machines. Thus while not eluding the Game of Life rules, new laws (computability theory) that are independent of the Game of Life rules apply at the Turing machine level of abstraction — just as Schrödinger said.

    “Furthermore, conclusions about Turing machines apply to the Game of Life itself. Because the halting problem is unsolvable, it is unsolvable whether an arbitrary Game of Life configuration will reach a stable state.

    “Not only are there independent higher level laws, those laws have implications for the fundamental elements of the Game of Life. I call this downward entailment, a scientifically acceptable alternative to downward causation.”

    — Russ Abbott, “Reductionism, emergence, and levels of abstractions”

  • floodmouse

    “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.” – Agreed. Just because I know I am using my prefrontal cortex to make a decision, that doesn’t mean I’m not deciding.

    “It’s only when you start asking “what effect do my feelings have on my protons and neutrons?” that you start getting syntax errors.” – Disagree. “Feelings” cause measurable changes in body chemistry, and “feelings” can be altered by conscious choice; otherwise, biofeedback wouldn’t work. Reference the idea of “mind-body integration” as taught by yoga.

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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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