PLEASE DON'T SAVE A COPY WHILE THIS NOTICE EXISTS
(TEMPORARY VERSION, STILL CHANGING)
NOTES BY Aaron Sloman (Aaron) ON COMMENTS BY Anthony Leggett (Tony)
Tony's Comments were pre-conference responses to my pre-conference
notes for my invited talk on 21ST JAN 2023 at the
14TH INTERNATIONAL WORKSHOP ON NATURAL COMPUTING, TOKYO, JAPAN
https://www.natural-computing.com/iwnc-ws
Invited speaker: Aaron Sloman (Aaron):
http://www.cs.bham.ac.uk/~axs
Invited responder:
Anthony Leggett (Tony):
https://physics.illinois.edu/people/directory/profile/aleggett
Tony was invited to comment on my (Aaron's) presentation, but was unable to join the conference at the time of presentation, so he provided some notes written in advance in response to my pre-conference draft presentation.
Tony's notes (quoted in a modified form below) were read out by the conference chair, Marcin Schroeder, after my presentation.
Pre-conference notes for my talk (later modified/extended below) were made available at
https://www.cs.bham.ac.uk/research/projects/cogaff/misc/evo-devo.html
which also includes a link to Tony's pre-conference notes.
My talk-notes have been revised since the conference, with more detail added,
below.
That evo-devo.html site now also includes a link to recordings of the presentations and discussions and a link to this [not necessarily final!] new document (begun 10th April 2023, partly delayed by computer problems).
This file is:
https://www.cs.bham.ac.uk/research/projects/cogaff/misc/leggett-sloman-evo-devo.html
I have managed to produce a PDF version which, for some reason does not include
the last few lines of this file after "NOTE BY Aaron"
https://www.cs.bham.ac.uk/research/projects/cogaff/misc/leggett-sloman-evo-devo.pdf
______________________________________________________________________________________
This new document is Aaron Sloman's (long-delayed) response to Anthony Leggett's
pre-conference notes on Aaron's original pre-conference document (which also
includes ideas presented in earlier talks). That pre-conference document is
subsumed in this new (April 2023) version (delayed by serious home computer
problems, following a 'crash' after the presentation in January -- the ultimate
cause of the crash was a collection of mistakes I made in setting up my computer
when it was new in 2020!!).
Tony's original response, written in January 2023, before the conference,
is available at
https://www.cs.bham.ac.uk/research/projects/cogaff/misc/tony-leggett-talk-notes.txt
Those notes, are included below in black and white text, interspersed with comments/responses (in blue and white) by me written in April.
In this new document, contributions by the two participants are indicated by
Tony (Anthony Leggett, all written before 21st Jan 2023)
and
Aaron
(Aaron Sloman, all text in blue, mostly written on or after 10th April 2023,
though based on earlier work between September 2020 and April 2023. This
includes my pre-conference notes and also some very recent elaborations and
extensions (in April), partly triggered by Tony's comments and questions. I
apologise for the length and complexity of the document -- due mainly to the
complexity of the problems and ideas.)
Note: an earlier version of this document labelled the discussants as "AL" and
"AS". It's possible that some of those occurrences have not yet been updated!
______________________________________________________________________________________
Notes by Tony (Anthony Leggett) in January 2023:
Over the last two years, and in his talk at this conference, Aaron has called
attention to a previously little-remarked phenomenon, namely the development in
vertebrate eggs, with only the crudest of external physical input, not only of
enormous physical differentiation but of competences in the newly hatched chicks
which are clearly impossible to explain as the result of experience.
Aaron
Note: although I started by discussing chicks (i.e. newly hatched birds) I also
referred to other species, e.g. alligators, crocodiles, turtles, snakes of
various kinds, i.e. species with very varied physical forms, physiological
materials -- e.g. feathers, scales, shells, etc., -- and also differing
environments and behaviours. In part it is this enormous diversity in a variety
of dimensions (also noted long ago by Lynn Margulis -- mentioned below) that
poses challenges for theoretical physics.
Tony
He has conjectured inter alia that the existing laws of physics may be inadequate to explain this phenomenon. The following brief remarks are a preliminary and no doubt crude attempt to put some "boundary conditions" on this speculation.
Before attempting to address the main question,let me address briefly a subsidiary one:Is chemistry "different"?
If I understand the relevant passages in his talk correctly, Aaron seems to be
opposing the "continuous" evolution described by the laws of physics to the
"discrete" events (presumably reactions. etc.) which are (part of) the province
of chemistry.
===
Aaron
Partly inspired by his book, I assume that this continuous/discrete mixture is
crucially important for various aspects of biological evolution and development,
though I don't claim to understand most of the details of developmental
continuity and discontinuity in space and/or time.
I would welcome help formulating that point more clearly, or more accurately or
both. Or pointing out that I am hallucinating non-existent problems!]]
============================
The above points form part of the background to my thoughts presented in talks
since late 2020, and they may or may not help readers to understand what I am
trying to say, below! Unfortunately, I have never asked Tony for comments on
my interpretation of Schrödinger: somehow the topic did not, as far as I
recall, turn up in our discussions.]]
We seem to have had a mis-communication here, as my notes and presentation were
unclear, partly because of the complexity of the topic, and partly because not
all my background assumptions were mentioned in our discussions leading up to
this conference.
I have not assumed that laws of physics refer only to continuous processes.
In particular, after reading Erwin Schrödinger's 1945 Book: What is life?, on
which I have comments on extracts here:
http://www.cs.bham.ac.uk/research/projects/cogaff/misc/schrodinger-life.html
(mostly written several years ago)
I thought he [ES] had pointed out that biochemical processes, all of which are
physical processes, include a mixture of
-- continuous processes (in which spatial relations between particles change,
e.g. moving closer together and further apart, and/or varying relative spatial
orientations or directions of motion)
and
-- discrete processes (in which chemical bonds between particles are
formed or removed, i.e. bonds that can restrict relative motion or orientation,
and which can also change physical/chemical properties of molecular structures).
So it seems to me that Schrödinger rejected versions of theoretical physics
that require all changes to be continuous (e.g. Newtonian mechanics, apart from
elastic collisions?) or require all changes to be discrete, e.g. because space,
or space-time(?), is assumed to be composed of something like a huge array or
network of distinct "points".
It is possible that I seriously misunderstood what Schrödinger wrote!
Discrete space-time seems to me to pose serious problems for rotation of large
rigid objects, with objects further from the centre of rotation (or axis of
rotation)
EITHER having to go through larger jumps between spatio-temporal
locations the further they are from the centre -- or axis -- of rotation, so
that spatial relations between parts of large rotating objects must become
increasingly distorted the further they are from axes or points of rotation
OR having to go through larger numbers of small jumps between S-T locations
composing the trajectories of particles further from the centre of rotation,
which will require many discrete changes of relationships between particles
during what we think of as rigid rotation, but which cannot be rigid at the
lowest size levels if space-time is discrete.
============================
Tony (in his original message, responding to some earlier notes on hatching processes by Aaron):
As a physicist,I think my reaction to that proposition is somewhat
ambivalent: there exists a very well - developed body of literature which
explains a large variety of chemical reactions in physical terms,
... (continued below)
Aaron
I don't doubt any of that, but I wonder whether that "large variety" is
sufficiently large and sufficiently varied to include the particular
chemistry-based bio-developmental processes in eggs that I completely failed to
notice until late 2020, and which turned out to be increasingly complex and
varied (during the following two and a half years) the more closely I looked at
fairly well-known biological facts regarding reproductive processes in
vertebrate eggs (and other more or less closely related processes and mechanisms
in other species). I'll try to give examples below -- partly inspired by the
ideas of Lynn Margulis on "symbiogenesis". Unfortunately, she
died while her work was unfinished.
A useful overview is in this obituary:
https://www.theguardian.com/science/2011/dec/11/lynn-margulis-obituary
Tony
... (continuing from above)
which explains a large variety of chemical reactions
in physical terms, in particular (though not only) by the quantum-mechanical
phenomenon of quantum tunnelling (transmission through a region of space where
the potential energy exceeds the original total energy of the reacting
particles), and in these calculations as such there is no point at which any
"discrete" event appears to take place.
Aaron
Does that imply that, in those contexts, chemical bond formation and release are
not discrete events that can produce discrete changes in relative motions, e.g.
a new bond preventing further relative motion, which is later made possible by
removal of the bond?
Tony
On the other hand, one could view chemical reactions as presenting just a
special case of the much more general (and notorious) "realization" (or
conventionally "measurement") problem of quantum mechanics, namely that in order
to make contact with experimental reality one has to go over at some point from
the quantum description in terms of co-existing alternative "realities" to the
classical notion that things either do or don't happen, and the terminal point
of a reaction seems as good a place as any to do so.
Aaron
What does that imply about the "realities" of changing relations between
(particle/minimal) components of increasingly large object-pairs, or triples,
etc. involved in relative spatial rotations?
Does/Do one, or the other, or both, ... or all, of the compound objects (objects
composed of particles joined by chemical bonds) have to undergo shape change
during relative spatial rotation?
Or is my question ill-formed??
I would like to see the implications of that remark expanded:
Tony
(There are strong and I believe credible arguments that postponing the
"realization" to a later point, such as the emission or not of a photon or the
registration of the latter by a macroscopic counter, does not affect the
predictions for the theory as regards probabilities of experimental outcomes).
Aaron
These are not questions about outcomes of human experiments but about outcomes
of chemical changes in biological processes. (Nature's experiments?)
Is there a corresponding point that can be made about formation or release
of a chemical bond?? How could all that be applied to physiological developments
occurring at different locations in an egg that need to be coordinated, e.g. so
that growing items meet where they need to meet, or so that appropriate
relations are created between growing structures (leading to more or
less serious physical deformities when the processes go wrong)?
Tony
So one could say that in this sense it is indeed legitimate to stress the above
opposition.
I think a useful way of approaching the main problem may be by analogy with a rather similar one which I believe arises in connection with another area of research (real or putative, depending on one's point of view), namely into the class of alleged phenomena usually labelled "psychical".
(Needless to say, a major difference is that in the present case, unlike that one, the existence of the phenomena themselves is not in dispute).
Let us imagine,purely for the sake of the present argument, that we believe, or would like to believe. that a given psychical phenomenon is real.
Aaron
Or a chemical control process in a hatching egg detecting an opportunity for a
new chemical bond to be formed, or an old bond to be released??
Could changes in one part of a developing embryo be controlled by detection of a
change in another part?
Could growth of a new structure be partly controlled by (remote) detection
of a gap towards which it should grow?
Can detected changes in relationships between components of the hatching embryo
cause changes in developments in other locations -- e.g. detection of a gap
causing growth of a new physiological structure towards the gap, or detecting a
new relationship between newly formed objects that makes possible, and
desirable, a new physical connection between them, e.g. connections between
developing bones and muscles and developing nerve fibres needed to control those
muscles in order to control motion of the bones?
Is it fair to claim that already known facts about physiological development in
eggs (at least vertebrate eggs, but perhaps also many other species, e.g.
insects undergoing metamorphosis in cocoons) imply that "proximal" interactions
between adjacent particles cannot suffice to control the construction of complex
physiological components with complex functions, in those species?
Insects emerging from cocoons typically not only have totally different
physiological sub-structures from their earlier ("grub") forms, e.g. wings and
legs, but also have totally different behavioural competences, such as flying,
feeding off nectar-producing plants, and mating. Some (e.g. migrating
butterflies) even migrate over long distances in their new state. How is the
information controlling those behaviours encoded in the chemical genome, and how
is the encoded information "extracted" and used at appropriate stages in the
individual's life?
Compare this with a physicist or a mathematician noticing a new implication
of a previously formulated theory??
Tony
Then we can raise the question: Is that phenomenon consistent with the laws of
physics as we currently know them?
(for the purposes of the thought-experiment, only, we assume that this current
knowledge is complete).
Aaron
I suspect not!
Would that imply that answers to all my questions about how embryo
development is controlled are known, or can be mathematically derived from
what is already known???
Tony
My rough-and-ready approach to answering this question (Is that phenomenon
consistent with the laws of physics as we currently know
them?) classifies the relevant phenomena into three categories, as follows:
(1) Does the phenomenon in question appear to violate the first law of thermodynamics?
If yes,then forget it (example: most types of levitation);
If no,continue.
(2) Does it appear to violate the second law?
If yes, then treat it with extreme scepticism, but be aware that the form which the second law takes in specific circumstances may be quite subtle (prima facie example: precognition);
If no,continue.
Aaron
E.g. control processes co-ordinating physiological
developments in different parts of an embryo, such as growth of nerve fibres or
blood vessels, towards their "intended" end locations, apparently moving
from a more random to a (much!) less
random distribution of particles.
Would release of chemical energy in related parts of the egg, or a supply of
heat energy by the "sitting" mother or ambient sunshine, provide all the energy
required to avoid conflict with the second law? Would that be an example of
Tony's case (3) below?
My presentation also mentioned far more "sophisticated" (is there a better
word?) transitions during gene expression
More subtly, we need to include the (presumably increasingly sophisticated)
later evolved gene expression mechanisms that orchestrate the transitions from
earlier forms of gene expression to later, more recently evolved, forms across
numbers of transitions that depend on the evolutionary history of the species.
(Is that summary cognitively digestible?? What does the ability of at least
some humans, or perhaps all, if suitably educated???, to think about such
things, add to what needs to be explained?)
Do Tony's questions subsume mine, or has he been (unintentionally) considerably
over-simplifying the specification of what needs to be explained?
Note:
Could *remote* (energy-free or not) control of sub-microscopic interactions
during any of the reproductive processes mentioned above violate the second law?
from:
gene-expression processes that evolved early in the history of the species and
take place during early phases
of in-egg development, e.g. where there is still relatively little
differentiation in the egg
to:
gene expression processes that evolved later in the history of the species and
occur during later phases of in-egg development involving coordination between
more complex structural developments (bones, muscles, tendons, nerve-fibres of
different kinds with different functions, arteries, veins, capillary networks,
glands, digestive mechanisms, waste-disposal mechanisms, temperature-control
mechanisms, species-specific outer coverings (scales, shells, feathers, etc.),
limbs (but not in snakes), tails, in some cases, etc. etc. etc...
I suspect that if Lynn Margulis were alive and reading this she would find none
of it surprising, even if she had never used similar examples or forms of
expression.
Tony
(3) Does it apparently not conflict with either the first or second law,but
nevertheless have no obvious explanation within the more specific laws (e.g. of
electromagnetism) which characterize our current paradigm? If yes,then keep an
open mind on it (example: water-divining,and probably some forms of telepathy);
if no,then we would not normally count the phenomenon as "psychic".
Aaron
Would my suggested forms of remote control of physiological structure formation
in a developing embryo in an egg also be examples of "having no obvious
explanation with specific laws", or do known laws suffice to provide
explanations of all such in-egg control processes?
Tony
Let's attempt to apply a similar triage to the phenomenon of
egg-hatching.
(1) Does it appear to violate the first law of thermodynamics? I think (and I assume Aaron would agree) that the answer is fairly obviously no; the process poses no problem for the conservation of the total energy of the universe.
(2) Does it appear to violate the second law? That may be a little more tricky:in the crudest and most obvious sense the answer would seem to be no, since to an order of magnitude the thermodynamic entropy in units of Boltzmann's constant k_B is simply the number of molecules n, which is also (unsurprisingly) the order of magnitude,in bits, of the information storable in the (thermodynamically accessible) states of these molecules, and the total entropy throughput from and to the environment is (taking the egg of a domestic chicken,with incubation period ~3 weeks, as an example) of the order of a few hundred or thousand times this.
Aaron
is that illustrated by the heat energy provided by a mother bird sitting on
eggs, or perhaps the sun providing heat energy to eggs of sea-turtles laid in
sand on a beach?
Tony
However, see below.
(3) Does it have no obvious explanation within our current paradigm?
Aaron claims so, and I think makes a plausible case;
Aaron
Note: When I first started asking Tony questions about hatching processes in
2020 I thought there might be some conflict with the second law of
thermodynamics, but later recognized that there are deeper problems about how
spatially separated processes are coordinated and how evolutionary processes
can produce eggs that are able to move from relatively simple forms of local
control of biochemical changes to increasingly complex forms of control acting
across longer distances as the developing embryo acquires more structurally and
functionally differentiated parts requiring development to be coordinated across
longer distances.
The problems discussed here in relation to hatching processes are far more
complex biochemical engineering problem than Maxwell's Demon's problem of
separating particles moving at speeds above and below a certain threshold into
two separate enclosures, though perhaps that can be seen as a very simple
special case of a very rich class of problems of control of sorting/arranging?
His demon detects and uses kinetic energy that particles already have. The
"hatcher demon" may have to provide particles or molecules with energy required
to transport them to where they are needed to form more complex molecules in
part-built structures.
Detecting the types of particles required in particular places is a far more
challenging task than comparing the speed of a moving particle with a threshold.
The task is even more complex if particles have to be assembled in one place
then transported to another place where they are needed, e.g. molecules
transported in arteries? Are the functions of veins (e.g. disposing of carbon
dioxide and other "waste" products) yet another class of control problems?
So the high level phenonomenon that I claim/conjecture/hypothesize has no
obvious explanation within our current paradigm is how all the *very detailed*
control is achieved of coordinated developmental changes of *increasing many*,
*increasingly distant*, spatially separated-but-functionally-intricately-related
physiological substructures, with different physiological functions.
Moreover, those control problems and mechanisms are not uniform:
I also suggest that discrete evolutionary changes (a key feature of sexual
reproduction based on discrete mating events) are needed to produce discrete
changes in the structures formed during the hatching process, and, less
obviously, corresponding discrete changes in the problems of controlling and
coordinating their development, requiring discrete changes in the solutions to
the problems of controlling reproduction and development.
This point is emphasized in the work of Lynn Margulis on "symbiogenesis", which
I encountered in the 1980s, including an Oxford Symposium focused on her ideas
in 2009, part of which is recorded in
Unfortunately, I have not been able to locate the video of the main presentation
by Margulis herself in Oxford.
See also (mis-spelling is on the website!):
Moreover, I suspect that discrete developmental changes in which there are
discrete changes of structure and function are difficult (or impossible?) to
represent in the most popular mathematical notations used by scientists,
engineers and logicians!
Can they be represented using differential calculus and integration with
logical operators?
That's related to the distinct layers of "bootstrapping" of control of
development of a fertilised egg presented in my talk (and very obscurely
presented in my diagrams).
[More detail needs to be added -- we need to collect and compare a variety of
different examples, corresponding to different stages of evolution and
different stages of development in less or more evolved species, very crudely
illustrated in the diagrams used in my presentation, e.g.
Early stages of control involve relatively few interacting particles in a
relatively small space in the egg close to the nucleus, whereas later stages of
control involve coordination of much larger numbers of interacting particles, or
larger, increasingly complex, previously synthesised, biochemical
sub-structures, separated by greater spatial distances, changing in coordinated
ways.
http://www.voicesfromoxford.org/historic-post-lynn-margulis/
I believe there are three videos of presentations on that day, but I've been
able to find only the first two. If you have a link for the main presentation by
Margulis please contact me! [a-sloman-at-bham-ac-uk]
Thanks!
https://www.theguardian.com/science/2011/dec/11/lynn-margulis-obtiuary
http://www.cs.bham.ac.uk/~axs/fig/evo-devo/evo-devo-final.jpg
]
Tony
I would comment however that one of the most interesting advances over the last
few decades in the area of physics which sometimes goes under the name of
"physics of complex systems" has been the realization that quite simple rules
for the evolution of a physical system, as modelled e.g.by cellular automata,
can lead to surprisingly complicated and sophisticated outcomes.
So I suspect that most physicists would guess that there is no fundamental difficulty regarding the evolution of a vast variety of different physical structures (which of course is quite different from saying that we know how it works!)
Aaron
I wonder whether any of those physicists have thought about the specific types of variety of structures and processes involved in the hatching processes that I have been describing in eggs, including variations across stages of development, variations across parts of a developing embryo, variations across evolutionary stages, variations across species evolving in different environments with different resources, opportunities, risks, dangers, etc. and of course variations across different evolved physiological structures and mechanisms, and different post-hatching behaviours, opportunities, threats, etc.
Tony
What about the appearance of various non-experience-based competences in newborn chicks?
Suppose we focus on a simple example such as the ability to avoid a solid obstacle.
An obvious question is: Is it possible to build a robot which can reproduce this competence?
If so,is the amount of information that the robot needs to have consistent with the information storage capacity of the egg, and if yes,is this so by a large margin?
Aaron
My questions have not focused on amount of storage required, but on types of
information stored.
The questions are not so much about the requirements for a specific agent with
specific capabilities, but requirements for evolution across multiple
generations of increasingly sophisticated agents of increasingly varied types in
increasingly varied environments. (Where "increasingly varied" refers to changes
across evolutionary generations in different evolutionary branches, rather than
changes in an individual's environment and behaviours.)
Tony
I strongly suspect that the answer to all three questions is yes (indeed, I would be very surprised if such robots do not already exist, for military if not for civilian applications).
Aaron
But that does not answer my questions, above. It's to some extent not a question
about a particular technology can achieve, but a collection of questions about
what varieties of self-extending technologies can and cannot achieve, and how
they achieve what they do achieve.
I do not believe any military or non-military technology exists that can
replicate, or even come close to replicating, the processes of in-egg
development of a chicken, an avocet, a sea-turtle, or any of the other
vertebrate species whose eggs produce fully formed (though under-developed) new
individuals with significant behavioural competences ready for use after
hatching.
Tony
As regards the second question, one would need to consider the number of pixels,etc., whose output would be in practice required to give a "reasonable" definition of "solid" (in appearance) and "obstacle".
(Yes, of course it will always be possible to confuse the robot by making the appearance of the object ambiguous, but I imagine that is also true of the chicks; indeed, I would be surprised if there is not a subset of the animal-psychology community who have fun doing just that).
And as regards section 3, well, as above, a naive order-of-magnitude estimate of the information storage capacity is the number of molecules n, which is of order 10^23, in this context a very large number. so even if one has to accommodate multiple competences, it is not obvious that when the problem is formulated in this way there is a problem with information storage capacity.
But what if (as I suspect Aaron might argue) the problem simply cannot be formulated in these crude terms of (quantifiable) information and entropy?
Aaron:
YES -- notice the growing diversity of species-specific types of problem that
need to be solved during development of the embryo and the corresponding
diversity of types of solution required during evolution of the species.
[[I need to expand on this]]
Tony
What if the competence to avoid an obstacle cannot be reduced simply to the
reaction to a particular class of pixel intensity distributions, but requires
something more,something we might call "understanding"?
(this conjecture may look more plausible in more geometrically or topologically sophisticated examples).
Aaron:
Many of the problems faced by new hatchlings cannot be expressed in these terms
(or using any of the standard paradigms of pattern recognition or visual
perception in psychology, neuroscience or AI).
For example, the biochemical processes producing newly hatched sea-turtles have
to give them the motivation and competences required to climb out of their
egg-shells, clamber onto the surrounding sandy surface and head in the direction
that will eventually take them to the deep waters where they can feed, mate,
reproduce, etc., dealing with various types of problem on the way (not all
successfully, of course).
How is all that information encoded at various stages in the development of
the new individual during the hatching process, and what roles can the
genetically provided information play in influencing the biochemical processes
appropriately so as to produce the required end results, including all the
physiological structures and mechanisms of the newly hatched individual, and
also the motivations and competences required to use those (changing) structures
and mechanisms during the developmental processes leading up to and beyond
hatching?
Is it likely, or even possible, that all the types of informed control across
changing forms of complexity and size of developing physiological substructures,
needed during and after hatching, are special cases of already known types of
physical mechanism?
Partial failures/errors and partial repairs
On the contrary there are many examples of gene expression that diverge from
what could be described as "normal" for a species, many of which are fatal,
though some produce non-fatal but serious deformities that can interfere with
normal development and require considerable support from conspecifics (e.g.
family members or medical experts in the case of humans). Conjoined twins are a
well known sub-class with several variants. The ability of such developmental
errors to be non-fatal may be connected with the kinds of variability in gene
expression in a species that are required for long term evolutionary changes to
occur.
Plant and animal breeding technologies depend essentially on such variability in
the mechanisms of gene expression.
More generally: it seems clear that without such variability, including some
more complex forms of genetic change than I have mentioned, evolution of the
variety of species that exist on our planet could not have occurred.
Moreover, as as Margulis and others have emphasised these evolutionary processes
include essential forms of collaboration between separately evolved organism.
A familiar example:
Humans could not exist without bacteria.
I hope I have not given the impression that I think the mechanisms I've been
talking about always work properly.
Tony
Then I suspect all bets are off,and I am not sure that as of now I have anything
useful to say about whether or not one would then require some major
generalization of the laws of physics to accommodate the "egg-hatching"
dilemma; one would probably need to be able to formulate the idea of
"understanding" in algorithmic terms, which I am not sure that I (or anyone)
know(s) how to do.
At any rate this whole complex of issues seems well worth puzzling about further, and I think that Aaron deserves our thanks for raising it.
NOTE BY Aaron
Comments, criticisms and suggestions are welcome, including comments pointing out that I am seriously mistaken, or that I've misunderstood some of Tony's comments!
(Email a dot sloman at bham.ac.uk)
Maintained by Aaron Sloman
School of Computer Science
The University of Birmingham
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