Strong and Weak Emergence – Consc

 

 

Strong and Weak Emergence

David J. Chalmers
Philosophy Program
Research School of Social Sciences
Australian National University

1 Two concepts of emergence

The term ‘emergence’ often causes confusion in science and philosophy, as it is used

to express at least two quite different concepts. We can label these concepts strong

emergence and weak emergence. Both of these concepts are important, but it is vital to

keep them separate.

We can say that a high-level phenomenon is strongly emergent with respect to a

low-level domain when the high-level phenomenon arises from the low-level domain, but

truths concerning that phenomenon are not deducible even in principle from truths in the

low-level domain.1 Strong emergence is the notion of emergence that is most common in

philosophical discussions 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 the high-level phenomenon arises from the low-level domain, but 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

In (P. Clayton and P. Davies, eds.) The Re-emergence of Emergence (Oxford University Press, 2006). Most of
this chapter was written for discussion at a Granada workshop on emergence, sponsored by the Templeton
Foundation. One section (the last) is modified from a posting to the Usenet newsgroup comp.ai.philosophy,
written in February 1990. I thank the editors and the participants in the Granada workshop on emergence for
their feedback.
1 In philosophers’ terms, we can say that strong emergence requires that high-level truths are not conceptually or
metaphysically necessitated by low-level truths. Other notions in the main text can also be formulated in these
modal terms, but I will mainly talk of deducibility to avoid technicality. The distinction between conceptual

discussions 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 and its relation to strong 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 two notions.

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 cases 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 time (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

and metaphysical necessity will not be central here, but in principle one could formulate finer-grained notions of
strong emergence that take this distinction into account.

in complex biological systems, for example, does not on its own threaten the completeness of

the catalogue 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 an

ultimately physicalist picture of the world need not be overly reductionist, but rather can

accommodate all sorts of unexpected richness at higher levels, as long as explanations are

given at the appropriate level.

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

In what follows, I will say a little more about both strong and weak emergence.

simple laws.

2

Strong 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 position 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

colourblind scientist given complete physical knowledge about brains could nevertheless not

deduce what it is like to have a conscious experience of red. Secondly, 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 his or her 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 physically identical to me might have no

consciousness at all. This suggests that the lawful connection between physical processes and

consciousness is not itself derivable from the laws of physics but is instead a further basic

law or laws of its own. The laws that express the connection between physical processes and

consciousness are what we might call fundamental psychophysical laws.

I think this account provides a good general model for strong emergence. We can think of

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 catalogue of physical facts about the world, supplemented by a complete catalogue

of facts about consciousness, a Laplacean super-being 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 these domains 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). In short, with the exception of consciousness, it

appears that all other phenomena are weakly emergent or are derived from the strongly

emergent phenomenon of consciousness.

One might wonder about cases in which high-level laws, say in chemistry, are not

obviously derivable from the 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

maintain 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 super-being 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 (since 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. Downward causation means that higher-level phenomena are not only irreducible

but also exert a causal efficacy of some sort. Such causation requires the formulation of basic

principles which state 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, it follows that low-level laws

will be incomplete as a guide to both the low-level and the high-level evolution of processes in

the world.2

To be clear, one should distinguish strong downward causation from weak downward

causation. With strong downward causation, the causal impact of a high-level phenomenon on

low-level processes is not deducible even in principle from initial conditions and low-level

laws. With weak downward causation, the causal impact of the high-level phenomenon is

deducible in principle, but is nevertheless unexpected. As with strong and weak emergence,

both strong and weak downward causation are interesting in their own right. But strong

downward causation would have more radical consequences for our understanding of nature,

so I will focus on it here.

I do not think there is anything incoherent about the idea of strong downward causation.

I do not know whether there are any examples of it in the actual world, however. While it is

certainly true that we can’t currently deduce all high-level facts and laws from low-level laws

plus initial conditions, I do not know of any compelling evidence for high-level facts and laws

(outside the case of consciousness) that are not deducible in principle. But I think it is

possible that we will encounter some. (See Kim (1992; 1999) for some doubts.)

2 In such a case, one might respond by trying to introduce new, highly complex, low-level laws to govern
evolution in these special configurations, in the effort to make low-level laws complete once again. But the
point of this intermediate sort of emergence will still remain. It will just have to be rephrased, perhaps as the
claim that non-configurational low-level laws are an incomplete guide to the evolution of processes. See Meehl
and Sellars (1956) for related ideas here.

Perhaps the most interesting potential case of downward causation is in the domain of

quantum physics, at least on certain ‘collapse’ interpretations of quantum mechanics. On

these interpretations, there are two principles governing the evolution of the quantum wave

function: the linear Schrödinger equation, which governs the standard case, and a nonlinear

measurement postulate, which governs special cases of ‘measurement’. In cases of

measurement, the wave function is held to undergo a sort of ‘quantum jump’ quite unlike the

usual case. A key issue is that no one knows just what is the criterion for a measurement

taking place. Yet it is clear that for the collapse interpretation to work, measurements must

involve certain highly specific causal events, 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.

Both consciousness and the quantum measurement case can be seen as strong varieties of

emergence in that they involve in-principle non-deducibility and novel fundamental laws. But

they are quite different in character. If I am right about consciousness, then it is a case of a

strongly emergent quality, while if the relevant interpretations of quantum mechanics are

correct, then it is more like a case of strong downward causation.

In principle, one can have one sort of radical emergence without the other. If one has

strongly emergent qualities without strong downward causation, 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 strong downward causation without strongly 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 strongly emergent qualities and strong downward

causation together. If so, one has a situation in which a new fundamental quality is involved in

new fundamental causal laws. This last option can be illustrated by combining the cases of

consciousness and quantum mechanics discussed above. In the familiar interpretations of

quantum mechanics according to which it is consciousness itself that is responsible for wave-

function collapse, the emergent quality of consciousness is not epiphenomenal but plays a

crucial causal role.

My own view is that, relative to the physical domain, there is just one sort of strongly

emergent quality, namely, consciousness. I do not know whether there is any strong

downward causation, but it seems to me that if there is any strong downward causation,

quantum mechanics is the most likely locus for it. If both strongly emergent qualities and

strong downward causation exist, it is natural to look at the possibility of a close connection

between them, perhaps along the lines mentioned in the last paragraph. The question remains

wide open, however, as to whether or not strong downward causation exists.

3 Weak emergence

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. I think that

understanding weak emergence is vital for understanding all sorts of phenomena in nature, and

in particular for understanding biological, cognitive, and social phenomena, as is demonstrated

in many of the other chapters in this volume.

I gave a quick definition of weak emergence earlier. But it is more satisfactory to

understand the notion by example, and then attempt to analyze it. The concept of emergence

is often tacitly invoked by theorists in cognitive science and in the theory of complex

systems, in such a way that it is clear that a notion of other than the notion of strong

emergence is intended. We can take it that something like weak emergence is at play here, and

we can then use the examples to make sense of just what weak emergence comes to.