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