[[Written for the Templeton Foundation workshop on emergence in Granada, August 2002. Given the informal nature of the workshop, I haven't been especially careful with citations and such, but I should note up front that not much of what follows is fundamentally original with me. I hope that nevertheless there is something useful at least in the way I have put things together.]]
The term "emergence" has the potential to cause no end of confusion in science and philosophy, as it is used to express two quite different concepts. We can label these concepts strong emergence and weak emergence. Both of these concepts are extremely important, but it is vital to keep them separate. As far as I can tell, the papers for the Granada workshop are about evenly divided between papers on strong emergence and papers on weak emergence, so there is a danger of miscommunication here.
We can say that a high-level phenomenon is strongly emergent with respect to a low-level domain when truths concerning that phenomenon are not deducible even in principle from truths in the low-level domain. Strong emergence is the notion of emergence that is most common in philosophical discussion of emergence, and is the notion invoked by the "British emergentists" of the 1920s.
We can say that a high-level phenomenon is weakly emergent with respect to a low-level domain when truths concerning that phenomenon are unexpected given the principles governing the low-level domain. Weak emergence is the notion of emergence that is most common in recent scientific discussion of emergence, and is the notion that is typically invoked by proponents of emergence in complex systems theory. (See Bedau 1997 for a nice discussion of the notion of weak emergence.)
These definitions of strong and weak emergence are first approximations, which might later be refined. But they are enough to exhibit the key differences between the notion. As just defined, cases of strong emergence will likely also be cases of weak emergence (although this depends on just how "unexpected" is understood). But vases of weak emergence need not be cases of strong emergence. It often happens that a high-level phenomenon is unexpected given principles of a low-level domain, but is nevertheless deducible in principle from truths concerning that domain. The emergence of high-level patterns in cellular automata — a paradigm of emergence in recent complex systems theory — provides a clear example. If one is given only the basic rules governing a cellular automaton, then the formation of complex high-level patterns (such as gliders) may well be unexpected, so these patterns are weakly emergent. But the formation of these patterns is straightforwardly deducible from the rules (and initial conditions), so these patterns are not strongly emergent. Of course, to deduce the facts about the patterns in this case may require a fair amount of calculation, which is why their formation was not obvious to start with. Nevertheless, upon examination these high-level facts are a straightforward consequence of low-level facts. So this is a clear case of weak emergence without strong emergence.
Strong emergence has much more radical consequences than weak emergence. If there are phenomena that are strongly emergent with respect to the domain of physics, then our conception of nature needs to be expanded to accommodate them. That is, if there are phenomena whose existence is not deducible from the facts about the exact distribution of particles and fields throughout space and tie (along with the laws of physics), then this suggests that new fundamental laws of nature are needed to explain these phenomena.
The existence of phenomena that are merely weakly emergent with respect to the domain of physics does not have such radical consequences. The existence of unexpected phenomena in complex biological systems, for example, does not on its own threaten the completeness of the catalog of fundamental laws found in physics. As long as the existence of these phenomena is deducible in principle from a physical specification of the world (as in the case of the cellular automaton), then no new fundamental laws or properties are needed: everything will still be a consequence of physics. So if we want to use emergence to draw conclusions about the structure of nature at the most fundamental level, it is not weak emergence but strong emergence that is relevant.
Of course weak emergence may still have important consequences for our understanding of nature. Even if weakly emergent phenomena do not require the introduction of new fundamental laws, they may still require in many cases the introduction of further levels of explanation above the physical level, in order to make these phenomena maximally comprehensible to us. Further, by showing how a simple starting point can have unexpected consequences, the existence of weakly emergent phenomena can be seen as showing that a simple physicalist picture of the world need not be overly reductionist, but rather can accommodate all sorts of unexpected richness at higher levels.
In a way, the philosophical morals of strong emergence and weak emergence are diametrically opposed. Strong emergence, if it exists, can be used to reject the physicalist picture of the world as fundamentally incomplete. By contrast, weak emergence can be used to support the physicalist picture of the world, by showing how all sorts of phenomena that might seem novel and irreducible at first sight can nevertheless be grounded in underlying simple laws.
In what follows, I will say a little more about both strong and weak emergence.
We have seen that strong emergence, if it exists, has radical consequences. The question that immediately arises, then, is: Are there strongly emergent phenomena?
My own view is that the answer to this question is yes. I think there is exactly one clear case of a strongly emergent phenomenon, and that is the phenomenon of consciousness. We can say that a system is conscious when there is something it is like to be that system: that is, when there is something it feels like from the system's own perspective. It is a key fact about nature that it contains conscious systems; I am one such. And there is reason to believe that the facts about consciousness are not deducible from any number of physical facts.
I have argued this case at length elsewhere (Chalmers 1996, 2002) and will not repeat the case here. But I will mention two well-known avenues of support. First, it seems that a colorblind scientist given complete physical knowledge about brains could nevertheless not deduce what it is like to have a conscious experience of red. Second, it seems logically coherent in principle that there could be a world physically identical to this one, but lacking consciousness entirely, or containing conscious experiences different from our own. If these claims are correct, it appears to follow that facts about consciousness are not deducible from physical facts alone.
If this is so, then what follows? I think that even if consciousness is not deducible from physical facts, states of consciousness are still systematically correlated with physical states. In particular, it remains plausible that in the actual world, the state of a person's brain determines their state of consciousness, in the sense that duplicating the brain state will cause the conscious state to be duplicated too. That is, consciousness still supervenes on the physical domain. But importantly, this supervenience holds only with the strength of laws of nature (in the philosophical jargon, it is natural or nomological supervenience). In our world, it seems to be a matter of law that duplicating physical states will duplicate consciousness; but in other worlds with different laws, a system with the same physical state as me might have no consciousness at all. This suggests that the lawful connection between physical processes and consciousness is not itself derivable from physical laws, but instead involves further basic laws of its own. These are what we might call fundamental psychophysical laws.
I think this provides a good general model for strong emergence. We can think of paradigm strongly emergent phenomena as being systematically determined by low-level facts without being deducible from those facts. In philosophical language, they are naturally but not logically supervenient on low-level facts. In any case like this, fundamental physical laws need to be supplemented with further fundamental laws to ground the connection between low-level properties and high-level properties. Something like this seems to be what the British emergentist C.D. Broad had in mind, when he invoked the need for "trans-ordinal laws" connecting different levels of nature.
Are there other cases of strong emergence, besides consciousness? I think that there are no other clear cases, and that there are fairly good reasons to think that there are no other cases. Elsewhere (Chalmers 1996; Chalmers and Jackson 2001) I have argued that given a complete catalog of physical facts about the world, supplemented by a complete catalog of facts about consciousness, a Laplacean superbeing could in principle deduce all the high-level facts about the world, including the high-level facts about chemistry, biology, economics, and so on. If this is right, then phenomena in this domain may be weakly emergent from the physical, but they are not strongly emergent (or if they are strongly emergent, this strong emergence will derive wholly from a dependence on the strongly emergent phenomena of consciousness).
One might wonder about cases in which high-level laws, say in chemistry, are not obviously derivable from low-level laws of physics. How can I know now that this is not the case? Here, one can reply by saying that even if the high-level laws are not deducible from the low-level laws, it remains plausible that they are deducible (or nearly so) from the low-level facts. For example, if one knows the complete distribution of atoms in space and time, it is plausible that one can deduce from there the complete distribution of chemical molecules, whether or not the laws governing molecules are immediately deducible from the laws governing atoms. So any emergence here is weaker than the sort of emergence that I suggest is present in the case of consciousness.
Still, this suggests the possibility of an intermediate but still "radical" sort of emergence, in which high-level facts and laws are not deducible from low-level laws (combined with initial conditions). If this intermediate sort of emergence exists, then if our Laplacean superbeing is armed only with low-level laws and initial conditions (as opposed to all the low-level facts throughout space and time), it will be unable to deduce the facts about some high-level phenomena. This will presumably go along with a failure to be able to deduce even all the low-level facts from low-level laws plus initial conditions (if the low-level facts were derivable, the demon could deduce the high-level facts from there). So this sort of emergence entails a sort of incompleteness of physical laws even in characterizing the systematic evolution of low-level processes.
The best way of thinking of this sort of possibility is as involving a sort of downward causation. It requires basic principles saying that when certain high-level configurations occur, certain consequences will follow. (These are what McLaughlin 1993 calls configurational laws.) These consequences will themselves either be cast in low-level terms, or will be cast in high-level terms that put strong constraints on low-level facts. Either way, low--level laws will be incomplete as a guide to both the low-level and the high-level evolution of processes in the world.
(In such a case, one might respond by introducing new, highly complex low-level laws to govern evolution in these special configurations, allowing low-level laws to be complete once again. But the point of this sort of emergence will still remain: it will just have to be rephrased, by saying that non-configurational low-level laws are an incomplete guide to the evolution of processes. See Meehl and Sellars 1956 for related ideas here.)
I don't think there is anything incoherent about the idea of this sort of downward causation. (Jaegwon Kim [e.g. Kim 1992, 1999] argues against downward causation, but I'm not sure to what extent we disagree — something to discuss at the workshop.) I don't know whether there are any examples of it in the actual world, however. While it's certainly true that we can't currently deduce all high-level facts and laws from low-level laws plus initial conditions, I don't know of any compelling evidence for high-level facts and laws (outside the case of consciousness) that are not deducible in principle. Others may know more about this than me, however.
Perhaps the most interesting potential case of downward causation is in the case of quantum mechanics, at least on certain "collapse" interpretations thereof. On these interpretations, there are two principles governing the evolution of the quantum wavefunction: the linear Schrödinger equation, which governs the standard case, and a nonlinear measurement postulate, which governs special cases of "measurement". In these cases, the wavefunction is held to undergo a sort of "quantum jump" quite unlike the usual case. A key issue is that no-one knows just what the criteria for a "measurement" is; but it is clear that for this interpretation to work, measurements must involve certain highly specific criteria, most likely at a high-level. If so, then we can see the measurement postulate as itself a sort of configurational law, involving downward causation. Of course in this case the configurational law is in effect already built into involves emergent behavior. Both of these can be seen as "strong" varieties of emergence in that they involve in-principle nondeducibility and novel fundamental laws. But they are quite different in character. If I am right about consciousness, then it is a case of an emergent quality, while if the relevant interpretations of quantum mechanics are correct, then it is more like a case of emergent behavior.
One can in principle have one sort of radical emergence without the other. If one has emergent qualities without emergent behavior, one has an "epiphenomenalist" picture on which there is a new fundamental quality that plays no causal role with respect to the lower level. If one has emergent behavior without emergent qualities, one has a picture of the world on which the only fundamental properties are physical, but on which their evolution is governed in part by high-level configurational laws.
One might also in principle have both emergent qualities and emergent causation together. If so, one has a picture on which a new fundamental quality is itself involved in laws of "downward causation" with respect to low-level processes. This last option can be illustrated by combining the cases of consciousness and quantum mechanics discussed above, as in the familiar interpretations of quantum mechanics according to which it is consciousness itself that is responsible for wavefunction collapse. On this picture, the emergent quality of consciousness is not epiphenomenal, but plays a crucial causal role.
My own view is that there is just one sort of emergent quality (relative to the physical domain), namely consciousness. I don't know whether there is any emergent causation, but it seems to me that if there is any emergent causation, quantum mechanics is the most likely locus for it. If both sorts of emergence exist, it is natural to examine the possibility of a close connection between them, perhaps along the lines mentioned in the last paragraph. For now, however, I think the question remains wide open.
Weak emergence does not yield the same sort of radical metaphysical expansion in our conception of the world as strong emergence, but it is no less interesting for that. I think it is vital for understanding all sorts of phenomena in nature, and in particular to understanding biological, cognitive, and social phenomena. Others can address those issues better than I can, however. Instead, I'll conclude by attaching a something I wrote a number of years ago (as a graduate student in 1990) but never published. This was in effect a meditation on clarifying and refining the notion of weak emergence, as it applies to a number of familiar examples.
Emergence is a tricky concept. It's easy to slide it down a slippery slope, and turn it into something implausible and easily dismissable. But it's not easy to delineate the interesting middle ground in between. Two unsatisfactory definitions of emergence, at either end of the spectrum:
(1) Emergence as "inexplicable" and "magical". This would cover high-level properties of a system that are simply not deducible from its low-level properties, no matter how sophisticated the deduction. There is little evidence for this sort of emergence, except perhaps, in the difficult case of consciousness, but let's leave that aside for now. All material properties seem to follow from low-level physical properties. This is not usually the sort of "emergence" intended by people who invoke the notion in contemporary scientific discussions, but it is near enough to the neighborhood that it often leads to confusion.
(2) Emergence as the existence of properties of a system that are not possessed by any of its parts. This, of course, is so ubiquitous a phenomenon that it's not deeply interesting. Under this definition, file cabinets and decks of cards (not to mention XOR gates) have plenty of emergent properties — so this is surely not what we mean.
The challenge, then, is to delineate a concept of emergence that falls between the overly radical (1) and the overly general (2). After all, serious people do like do use the term, and they think they mean something interesting by it. It probably will help to focus on a few core examples of "emergence":
(A) The game of Life: High-level patterns and structure emerge from simple low-level rules.
(B) Connectionist networks: High-level "cognitive" behaviour emerges from simple interactions between dumb threshold logic units.
(C) The operating system (Hofstadter's example): The fact that overloading occurs just around when there are 35 users on the system seems to be an emergent property of the system.
(D) Evolution: Intelligence and many other interesting properties emerge over the course of evolution by genetic recombination, mutation and natural selection.
Note that in all these cases, the "emergent" properties are in fact deducible (perhaps with great difficulty) from the low-level properties (perhaps in conjunction with knowledge of initial conditions), so a more sophisticated concept than (1) is required. Another stab at a definition might be:
(3) Emergent = "deducible but not reducible". Biological and psychological laws and properties are frequently said not to be reducible to physical laws and properties. For many reasons, not the least being that the high-level laws/properties in question might be found associated with all kinds of different physical laws/properties as substrates. (A universe without protons and electrons might nevertheless include learning and memory.)
There are some problems with this definition, though. Firstly, it's not clear what is gained by trying to explicate emergence in terms of the almost-equally-murky concept of "reduction". Secondly, it seems to let in some not-paradigmatically-emergent phenomena, and it's not clear how some emergent phenomena like (A) or (C) would fit this definition. I think that (3) picks out a very interesting class, but it's not quite the class we're after. It's on the right track, though, I think.
The notion of reduction is intimately tied to the ease of understanding one level in terms of another. Emergent properties are usually properties that are more easily understood in their own right than in terms of properties at a lower level. This suggests an important observation: Emergence is a psychological property. It is not a metaphysical absolute. Properties are classed as "emergent" based at least in part on (1) the interestingness to a given observer of the high-level property at hand; and (2) the difficulty of an observer's deducing the high-level property from low-level properties. The properties of XOR are an obvious consequence of the properties of its parts. Emergent properties aren't. We might as well give this a number:
(4) Emergent high-level properties are interesting, non-obvious consequences of low-level properties.
This still can't be the full story, though. Every high-level physical property is a consequence of low-level properties, usually non-obviously. It feels unsatisfactory, for instance, to say that computations performed by a COBOL program are an emergent property relative to the low-level circuit operations — at least this feels much less "emergent" than a connectionist network. So something is missing. The trouble seems to lie with the complex, kludgy organization of the COBOL circuits. The low-level stuff may be simple enough, but all the complexity of the high-level behaviour is due to the complex structure that is given to the low-level mechanisms (by programming). Whereas in the case of connectionism or the game of life it feels that we have simplicity in both low-level mechanisms and their organization. So in those cases, we have much more of a "something for nothing" feel. Let's try for another number:
(5) Emergence is the phenomenon wherein complex, interesting high-level function is produced as a result of combining simple low-level mechanisms in simple ways.
I think this is much closer to a good definition of emergence. Note that COBOL programs, and many biological systems, are excluded by the requirement that not only the mechanisms but their principles of combination be simple. (Of course simplicity, complexity and interestingness are psychological concepts, at least for now, though we might try to explicate them in terms of Chaitin-Kolmogorov-Solomonoff complexity if we felt like it. My intuition is that this is likely to prove a little simplistic, although Chaitin has an interesting paper that attempts to derive a notion of the "organization" of a system using similar considerations.) And note also that most things that satisfy this definition should also satisfy (4) — due to our feeling that simple principles should have simple consequences (or else complex but uninteresting consequences, like random noise). Any complex, interesting consequence is likely to be non-obvious.
This does indeed fit in with the feeling that emergence is a "something for nothing" phenomenon — though in a more subtle and satisfactory way than set forth in (1), for instance. It's a phenomenon whereby "something stupid buys you something smart". And most of our examples fit. The game of Life and connectionist networks are obvious: interesting high-level behaviour as a consequence of simple dynamic rules for low-level cell dynamics. In evolution, the genetic mechanisms are very simple, but the results are very complex. (Note that there is a small difference, in that in the latter case the emergence is diachronic, i.e. over time, whereas in the first two cases the emergence is synchronic, i.e. not over time but over levels present at a given time.)
We're still not completely there — it's not clear how (C), the operating system example, fits into this paradigm of emergence. But throwing in a smidgen of teleology should get us the rest of the way. I.e., we have to notice that everything here has to be relativized to design. So we design the game of Life according to certain simple principles, but complex, interesting properties leap out and surprise us. Similarly for the connectionist network — we only design it at a low level (though in this case we hope that complex high-level properties will emerge). Whereas in the COBOL case — and in the case of much traditional AI — you only get out what you put in (N.B. I'm not necessarily knocking this: at least here, I'm trying to explicate emergence, not to defend it). And now the operating system example fits in well. The design principles of the system in this case are quite complex — unlike the other cases that fit (5) above — but still the figure "35" is not a part of that design at all. So:
(6) Emergence is the phenomenon wherein a system is designed according to certain principles, but interesting properties arise that are not included in the goals of the designer.
Notice the appearance of the word "goal" — this is important, any design is goal-relative. So the notion now is quite teleological. I notice that Russ Abbott makes a similar point in a recent posting. Notice, however, that as we've conceded that emergence is a psychological property, we're able to construe teleology in a psychological, non-absolute way. So for our purposes here, we only need the appearance of teleology. This is nice, because it allows us to include system where strictly speaking, "design" doesn't apply at all. In evolution, for instance, there is no "designer", but it is easy to treat evolutionary processes as processes of design. On more than one level.
We can view evolution as teleological at the level of the gene — as in Dawkins' theory, for instance. Then the appearance of complex, interesting high-level properties such as intelligence is quite emergent. We also can reconstrue evolution as teleological at the level of the organism (this is perhaps a more straightforward Darwinian view of things). On this construal, the most salient adaptive phenomena like intelligence are no longer emergent, but the goal of the design process. However, this view does open up the possibility of other kinds of emergent phenomena: firstly, non-selected-for byproducts of the evolutionary process (such as Gould and Lewontin's "Spandrels"); secondly and more intriguingly, it allows an explanation for why consciousness seems emergent. Raw consciousness may not have been selected for, but it somehow emerges as a byproduct of selection for adaptive process such as intelligence.
It's probably foolish to search for a definitive construal of "emergence": like most psychological concepts, it probably is best construed as a "family resemblance" — each of the "definitions" outlined above might play some role. Personally, I'm happiest with a combination of (5) and (6) — with (5) being the "core" variety of emergence, and (6) being a more general variety of which (5) is a special case.
Bedau, M. 1997. Weak emergence. Philosophical Perspectives 11:375-399.
Broad, C.D. 1925. The Mind and its Place in Nature. Routledge.
Chalmers, D.J. 1996. The Conscious Mind: In Search of a Fundamental Theory. Oxford University Press.
Chalmers, D.J. 2002. Consciousness and its place in nature. http://consc.net/papers/nature.html.
Chalmers, D.J. & Jackson, F. 2001. Conceptual analysis and reductive explanation. Philosophical Review 110:315-61. http://consc.net/papers/analysis.html/.
Kim, J. 1992. The nonreductivist's trouble with mental causation. In (J. Heil & A. Mele, eds) Mental Causation. Oxford University Press.
Kim, J. 1999. Making sense of emergence. Philosophical Studies 95:3-36.
McLaughlin, B.P. 1992. The rise and fall of British emergentism. In (A. Beckermann, H. Flohr, & J. Kim, eds) Emergence or Reduction?: Prospects for Nonreductive Physicalism. De Gruyter.