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From: dave@cogsci.indiana.edu (David Chalmers)
Newsgroups: comp.ai.philosophy
Subject: Thoughts on emergence
Message-ID: <62500@iuvax.cs.indiana.edu>
Date: 6 Oct 90 07:08:47 GMT
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Reply-To: dave@cogsci.indiana.edu (David Chalmers)
Organization: Indiana University, Bloomington
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Here are some thoughts on "emergence".  Nothing definitive, but an attempt
to get at the psychological core (or cores) of the notion.  Thanks are due to
others for providing a stimulating discussion.

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.  This view leads
easily into mysticism, and there is not the slightest evidence for it (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.  Very few sophisticated people since the 19th century have
actually believed in this kind of "emergence", and it's rarely what is referred
to by those who invoke the term favourably.  But if you mention "emergence",
someone inevitably interprets you as meaning this, causing no end of 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 deeply implausible (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.  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" (or "subjectivity" or
"qualia" or whatever) seems emergent.  Raw consciousness doesn't not seem to
have been selected for, as it doesn't play any direct functional role (though
it does have functional counterparts; this is a subtle issue, but remember
we're talking about the way things *seem*, not the way they are); 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.

--
Dave Chalmers     (dave@cogsci.indiana.edu)      
Concepts and Cognition, Indiana University.

"It is not the least charm of a theory that it is refutable."