Galileo concluded that large objects must fall at the same rate that small ones do by using an ingenious thought experiment. First he imagined two rocks, roughly the same size, dropped from some height, falling at whatever rate rocks of their size fall. Then he imagined that the experiment were repeated, this time with the rocks tied together with a piece of string. Are we really to imagine, he wondered, that Nature would suddenly regard the two rocks as one large object, and make it/them fall at a different rate just because they were now connected with a string? He reasoned that Nature would not. When does Nature regard things as actual, individual things, and when does nature regard them as heaps, or aggregates of other things, like Galileo's rocks tied together? And perhaps more importantly, in what sorts of situations would the answer make any difference at all?
For an honest hard-nosed reductionist, the universe is really a sea of quantum soup. There are no true inherent things, just one continuous mesh of cause and effect. Minds, and only minds, draw boxes and lines upon reality based on perceived regularities, chunking reality into mid-level things, like "rocks" and "cars". This chunking is an abstraction we impose, and is not there in the quarks, electrons, and photons. We could, in principle, see a certain number of molecules as a "rock", or we could just see it as a bunch of molecules with no loss of accuracy or predictive power.
"Reductionism" is a loaded term, and one that tends to get thrown around pejoratively. When I use the term, I will attempt not to make an unfair caricature or a straw man of it. Reductionism, very roughly, is the divide-and-conquer approach to understanding reality. It is the position that anything just is the sum of its parts. Now, sometimes reductionism means methodological reductionism, which is simply the practice of analyzing things in terms of their components. Methodological reductionism as an approach to scientific inquiry has been spectacularly successful over many centuries. When I speak of reductionism, however, I mean it in a stronger, ontological sense. I mean the presuppositions (and I believe that they are presuppositions)
Reductionism combined with deterministic physicalism results in the claim that if you knew the exact initial conditions of the universe, and knew the true laws of physics, you could, in principle, predict everything that would ever happen during the lifetime of the universe, including the fall of the Roman empire and the Gettysburg Address. There are no big, large-scale things that can not be understood fully in terms of their simpler, small-scale underlying mechanisms.
Many prevailing theories of mind incorporate some form of this strong ontological reductionism, even ones that make a point of claiming to reject strict reductionism. I think, however, we have reason to doubt that reductionism in this sense gives us a true or complete picture of the world. The problem with reductionism is that it works too well. If everything can be explained or characterized in terms of the lowest level building blocks, there is no reason to consider higher level things as having any objective existence at all, or at least, any explanatorily useful existence.
How can we have things in a reductionist universe? By things, I mean just what it sounds like: cars, dogs, planets, paper clips. Is a pile of sand a thing, or is it a lot of little things? Does a car count as a thing, or is it a lot of smaller things, all working together? It depends on how you look at it, and why you want to know. What things can there be in the world whose existence (as individual things) is not just a matter of perspective in this way? And do we have any reason to believe that there are any higher-level things in the world that just are the things they are, whether you look at them in the right way or not?1
If we are reductive materialists, then speaking absolutely objectively, there is either only one (extremely high-level) thing in the entire universe (the universe itself), or there are as many (extremely low-level) things as there are subatomic particles. There is no absolute reality to any intermediate level things as such.
It does not buy you anything (in terms of imparting thinghood) to declare certain systems as unitary wholes on the basis that they are isolated from their surroundings, because everything interacts with everything else all the time. This is not New Age mysticism, but simple fact. The force of gravitation between any two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them. This number is never zero for any two objects, no matter how small the masses or how great the distances involved.
I once read somewhere that the gravitational effect of an electron on the trajectory of a molecule of gas a universe away is such that after being amplified by about 50 collisions with other gas molecules, this tiny gravitational nudge is enough to cause the gas molecule's position to be off by the width of a entire molecule. This, in turn, determines whether or not the molecule collides with the next molecule at all or misses it entirely, a difference which quickly changes the dynamics of the entire volume of gas. Whether the correct number of collisions before this happens is really 50 or 50 million, there is some finite number of which this must be true. All particles in the universe interact causally with all others all the time (the contents of black holes possibly excepted).
But still, one might argue, there are some things which act more or less together as one, and are separable from their environment. Consider a toy truck. It seems thing-like if anything does. But just as the computer program does not "know" anything but the current machine code instruction, the truck is just made of atoms, each of which does not know or care anything about "truck" as opposed to all the other atoms that are "non-truck". Each atom only "knows" about the forces that act upon it, and each reacts accordingly. Each atom would still behave the way it does under the influence of any equivalent immediate environment (local to just that atom, that is) whether that environment was the result of that atom's participation in what we might be inclined to call a "truck", or some other, completely different system, as long as it presented the exact same interface to the atom. The atom does not act the way it does because of some high-level organization of the system of which it is a part.
A complete knowledge of the forces acting immediately upon each atom in the truck is all that is necessary to have complete and perfect knowledge of the patch of reality that we call "the truck". It gives us complete predictive power over all of the atoms involved, at any level of detail you like. In a completely objective reductionist universe, there is nothing to know about the truck above and beyond all these atoms.
If there were no people to observe it and think of it as a truck, and the truck were on a hill, kept in place by a slowly rotting chock block, and eventually the chock gave way and the truck rolled down the hill, we might be tempted to say that the truck behaved in a trucklike way on its own, thus manifesting its thinghood in contrast to its environment. But I could still argue that it did no such thing. There were atoms, and they each did what they did, acting upon each other each to each. Some bonds were weaker than other bonds, some atoms were more free to move with respect to their neighbors than others, and every aspect of what we, at a high level, would describe as the truck rolling as a unit down the hill is completely characterizable in these terms alone, without making a separate unit of the truck itself as such. We find trucks useful, so we draw a line around a patch of reality with a purple crayon and call it "truck", but there is no truckhood in the underlying physical reality.
It is sometimes said that higher level properties and thus higher level things emerge from the lower levels in a way that is not determined or even suggested by the lower levels. The flock emerges from the motions of the individual birds, liquidity emerges from the actions of trillions of H2O molecules. The claim that there is genuine emergence in the world is often contrasted with reductionism.
There are several flavors of emergentism (and the closely related theories of so-called nonreductive physicalism), but most of them, in my opinion, do not convincingly dig their way out from under reductionism as they claim to do. This is because emergence more often than not reflects nothing more than a cognitive limitation on our part. We are just not smart enough to infer the liquidity directly from a complete knowledge of the H2O molecules.
Nevertheless, there is no objective, measurable property of a bucket of water (including facts about the liquidity of the water) that one could not, in principle, infer given 1) a complete and perfect description of each atom of hydrogen and oxygen in the bucket (i.e. a complete set of initial conditions), 2) a complete and perfect set of physical laws that described the behavior of hydrogen and oxygen atoms through time as they interacted, and 3) the vast cognitive power it would require to model all those atoms and calculate their interactions.
In general, we are stupid - it is easier by far to frame our understanding of the world in high-level terms, to understand "water" as "sloshing" in certain ways, and even to come up with precise laws about the ways in which water sloshes. But this is just a shorthand way of describing what is actually the aggregate motion of trillions of molecules. This shorthand description does not tell us anything that could not, in principle at least, be derived from the trillions of molecules themselves - its advantage is that it is so much easier to deal with. As David Chalmers has pointed out, emergence is a psychological concept: it is a measure of our surprise at the consequences of low-level natural laws, not a fundamental truth of Nature in its own right. A bumper sticker slogan sometimes invoked by emergentists is "More is different". I would counter that more only seems different.
There are no high-level facts or properties that magically "emerge" only at the high level. Such purported properties are only convenient terms we use to describe low-level things like atoms and photons on a massive scale. Heat, for example, just is the aggregate mean kinetic energy of a collection of molecules. Any laws or knowledge we have of heat could in principle be derived from, or expressed in terms of, all of the facts about all the individual molecules involved in a given collection being considered.
I should emphasize that the ability, for example, to reduce chemistry to physics is an in principle reduction only. No discoveries in the field of physics will ever render chemistry (or biology, or sociology, etc.) obsolete as fields of legitimate inquiry. Even in a universe in which reductionism is absolutely true, the physical world is hugely complex, and its complexities explode out of control very quickly in a chaotic fashion without any hope of being modeled at the low levels by beings with cognitive limitations like us. It will always be astronomically easier to deal in terms of higher-level chunks of reality than in atomic terms or subatomic terms for almost all purposes. Nevertheless, in principle, if you could model reality at the low level in a reductionist's universe, that would be all you would need to derive any measurable fact about that universe. Any higher level chunking of reality is a cognitive convenience. Put differently, the universe has no need of any "high level" things or concepts as it clanks along one moment to the next. All of the explanatory heavy lifting is done at the lowest level.
More to the point, emergence (invoked in this way) strikes me as an attempt to dodge the Hard Problem by paying lip service to the idea of qualitative essences (like the liquidity of water, and higher-level properties in general) but placing the problem out there in the world, when it is really in here, in our minds. There is no liquidity in the world, except that which is directly inferable from the actions of the H2O molecules (in which case the "emergence" of liquidity from the actions of the molecules melts away as a concept capable of explaining anything), but there is a wetness quale in our minds.
What is mysterious about liquidity is not the objective, quantifiable behavior of the bucket of water or its relation to the motions of all the molecules, but the quale, the sense of wetness in our own minds. To the extent that high-level properties refer to objective measurable qualities in the external world, they are just shorthand ways of talking about the low-level properties that constitute them. To the extent that they are not inevitable logical consequences of low-level properties, they exist only in the mind.
The problem that emergence tries to solve or at least articulate is the Hard Problem that dare not speak its name. Proponents of most forms of emergentism and nonreductive physicalism are, in my opinion, trying to straddle the fence. On one hand, they have some inkling that strict reductive physicalism is inadequate to account for the universe as presented to us, but on the other hand, they are unable or unwilling to make the freaky metaphysical commitments that are necessary to address these inadequacies. They don't want to have to build any magic into the ground floor of their universe, so they try to slipstream it in somewhere in the middle. My argument is that we need real magic here, and all mid-level things in a reductionist universe are only may-be-seen-as kinds of things. The only magic you can slipstream into the mid levels, then, is may-be-seen-as magic. If you accept reductionism, or anything like it, you don't get to invoke "high-level" entities that carry any ontological weight.
In a purely reductionist universe, with no absolute thinghood above the subatomic level, no natural mid-level principles of individuation, and everything just more or less dense patches in the quantum soup, I imagine that the mind of God is like that of Neo at the end of the movie The Matrix. If you have not seen it, I urge you to do so - it is great fun very well done, and touches on some themes that are relevant to these essays (don't bother with the sequels). Much of the action in the movie takes place in an extremely realistic computer simulated reality ("the matrix" of the title). While the characters are really comatose in reclining chairs with data feeds plugged into the bases of their skulls sometime in the distant future, they perceive themselves to be walking, driving, fighting, etc. in late 20th century America. At the end of the movie, the hero, Neo, has a sort of awakening while in the matrix as he confronts the sinister Agents who want to kill him (virtually dying while in the matrix results in actual physical death). The final confrontation had a great special effect in that it captured the essence of an inherently non-visual idea and did so simply and clearly. Neo sees the outlines of the floor, the walls, the ceiling, and the three Agents, but all of their surfaces from his point of view are a wash of iridescent green computer characters, the same ones that were on the screens in the matrix's monitoring center back in physical reality. Neo sees through the matrix, stops accepting it on its terms, and sees straight down to the level of the data of which it is made. And of course, this essentially makes him God within the matrix.
In a reductionist universe, God (if there were God in a reductionist universe) sees everything this way. His mind tracks every last neutrino with perfect accuracy, and He does not have to use our shortcuts of chunking patches of reality into "whale", "bridge", "apple". It is only a consequence of our own perceptual and cognitive limitations that we find it necessary to chunk the universe into "flocks" or even individual "birds". In real life, there are no higher levels. The universe, to a reductionist, models or computes itself at the lowest of all possible levels. Once all the hydrogen and oxygen atoms follow their basic laws, there is neither any need for, nor room for, any further laws about "liquidity", "transparency", or any other high-level properties of water in order for the universe to "know" how water should behave instant to instant. The universe crunches along, doing what it must, not because of any patterns or any way in which such patterns are organized, or because of their purported complexity but because the particular quarks and electrons and so on with their particular positions and momenta must do what they must do. "Patterns" are a way of categorizing reality for us, a way of setting up a taxonomy of classifications of what are ultimately physical systems. You can't possibly get any magical new properties to "emerge" out of a collection of stuff because it is "complex", above and beyond what you would have gotten out of that same collection of stuff anyway. Anything that is really, really there at the high level must have been really, really there at the low level.
If, that is, we are committed reductionists.
1 Gregg Rosenberg has done some good thinking about this in his search for what he calls natural individuals.