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.