What might dynamical intentionality be, if not computation?


Commentary on van Gelder's ``The Dynamical Hypothesis In Cognitive Science"



Ronald L. Chrisley

School Of Cognitive  Computing Sciences

University of Sussex

Falmer  BN1 9QH

United Kingdom

ronc@cogs.susx.ac.uk

http://www.cogs.susx.ac.uk/users/ronc



(Words within "*" are to be italicized)



ABSTRACT



I make five points.  1) I note that van Gelder's concession that the

Dynamical Hypothesis is not in opposition to to computation in general

does not jibe well with his anti-computational rhetoric.  2) I dispute

his claim that dynamic systems allow for non-representational aspects

of computation in a way in which digital computation cannot.  3) I

distinguish two senses of the ``cognition is computation'' claim, and

point out that van Gelder argues against only one of them.  4)

I suggest that dynamical systems as characterized in the target

article suffer from the same problems of universal realizability as

formal notions of computation do, but differ in that there is no

solution available for them.  5) I show that the Dynamical Hypothesis

cannot tell us what cognition is, since instantiating a particular

dynamical system is neither necessary nor sufficient for being a

cognitive agent.







Given van Gelder's concession (in sections 6.3, 6.5 and 6.10) that he

is not opposing computation in general, just digital computation in

particular, I have no disagreement with his main point.  It is indeed

an open empirical issue which kind of computation best characterizes

natural cognitive agents.  However, I do object to the misleading way

in which he goes about stating this.  Yes, ``research into the power of

dynamical systems is an interesting new branch of computation theory"

(page 15).  But with that considerable concession in mind, van Gelder

shouldn't have thought he was rejecting effectiveness; he was only

pointing out that processes which are quantitative (at the ``highest

level") can be effective -- effectiveness need not imply digitality.

And he shouldn't have named the view he is opposing ``the computational

hypothesis" when it is really a specific form of digital computation

which is his target.



Although van Gelder wisely avoids the anti-representationalism that

has been the focus of some recent dynamical criticisms of

computational accounts of cognition, he fails to resist mentioning

anti-representationalism altogether (section 4.2.3.9). He is mistaken,

however, in thinking that only quantitative systems can accommodate

non-representational aspects of cognition.  For example,

Brooks (1992) has famously rejected representations in the

construction of mobile robots which behave intelligently in real time

in the real world, yet his subsumption architectures are not

quantitative, but rather are of the same kind as digital computational

architectures.  Perhaps it is right to reserve the term ``computation"

for processes that involve representations.  But then there is a

natural superclass of digital computation, let us call it the class of

``digital machines", which stands in the same relation to digital

computation as dynamical systems stand to dynamical

computation. Despite recent rhetoric, there is no reason to believe

that dynamical systems have any ``non-representational" advantage over

digital machines.



A distinction should be made between two senses of the claim

``cognition is computation".  One sense (call it the ``opaque

reading") takes computation to be whatever is described by our current

computational theory, and claims that cognition is best understood in

terms of that theory.  The transparent reading, by contrast, has its

primary allegiance to the phenomenon of computation, rather than to

any particular theory of it.  It is the claim that the best account of

cognition will be given by *whatever theory turns out to be the best

account of the phenomenon of computation*.  The opaque reading is a

claim about specific theories, while the transparent claim is a claim

about the phenomena of computation and cognition themselves.  The

``cognition is computation" claim can be true on the transparent

reading, even if cognition isn't best understood in terms of, say,

formal operations, just as long as such operations turn out not to be

good accounts of what makes actual computers work.  I'm one of those

people who believe formal notions of computation to be inadequate

theoretical accounts of actual computational practice and artifacts

(what Brian Smith (1996) has called ``computation in the wild").

van Gelder, however, insists (in section 6.5)

on opposing himself to the formal notion of computation.  This is

understandable, since the formal view of computation is the de facto

orthodoxy, and we are still waiting for a non-formal theoretical

alternative.  But if it turns out that what makes the artifacts of

Silicon Valley tick is not best explained in terms of formal

computation, then van Gelder's discussion will have nothing to say

against the transparent version of the ``cognition is computation"

claim.



But van Gelder's focus on formality in characterizing his opponent

seems to have the unfortunate consequence of causing him to

characterize dynamical systems as formal also.  A recurring criticism

of the computational approach is that its formality renders it

universally realizable --  Putnam (1988) and  Searle (1990) argue that

any physical system can be interpreted as realizing any formal

automaton.  This has the consequence that an account of cognition

cannot be in terms of formal computation, since any particular formal

structure, the realization of which is claimed to be sufficient for

cognition, can be realized by any physical system, including those

that are obviously non-cognitive. Dynamical systems as van Gelder

characterizes them, also seem to be universally realizable in this

sense -- one can employ Putnam's tricks to show that every physical

system instantiates every dynamical system.  But the difference is

that there is a known way out of this problem for digital computation,

while there is not for dynamical systems.  Since computation is not

purely formal, but includes an implicit notion of discrete states and

causal transitions between them, one can use this to restrict the set

of physical systems that can be properly said to instantiate any given

computation, thus avoiding universal realizability (Chrisley 1994).

But how are we to so restrict the set of

physical systems which realize any given dynamical system, without

rendering the dynamical system non-quantitative in the process?



van Gelder's response to the ``Not As Cognitive" objection (section

6.7) won't help him here.  What he says is correct: just as the

digital computation hypothesis does not claim that all digital

computers are cognizers, but rather that cognizers are a special kind

of digital computer, so also, mutatis mutandis, for the Dynamical

Hypothesis (DH).  The DH is not giving sufficient conditions for

cognition.  But it does claim that the sufficient conditions can be

given in terms of dynamical systems, as he has construed them.  And

the universal realizability points just made cast doubt on that.



Perhaps the universal realizability point can be countered for

dynamical systems, as it was for digital computational systems.

Nevertheless there is a difficulty that arises out of van Gelder's

admission that the DH is not providing sufficient conditions for

cognition: it puts all the weight on the other foot.  It implies that

the theoretical value of the DH must be in its providing *necessary*

conditions for cognition. But van Gelder admits that the DH is *not*

giving necessary conditions for cognition, either.  Since it takes no

stand on the nature of artificial cognition (section 4, paragraph 2),

the DH is not a constitutive claim about the essence of cognition in

general, but rather a contingent claim about natural cognizers.  Aside

from relying on a natural/artificial distinction which removes us and

our artefacts from the natural world, rather than seeing us/them

continuous with it, the DH has the drawback of leaving us without a

constitutive account of cognition.  The most likely place to look for

such an account is not in the particularities of natural cognizers,

but in the commonalities between all of the systems worthy of the

title: natural cognizers, natural cognizers in other possible worlds,

and (as yet hypothetical) artificial cognizers. E.g., what do (natural)

quantitative intentional effective systems and (artificial) digital

intentional effective systems have in common?  Intentional

effectiveness.  Perhaps, then, that is the true nature of cognition.





























REFERENCES:


Brooks, R. (1992) Intelligence without representation.  In:


   Foundations of Artificial Intelligence, ed. D. Kirsh.  MIT Press.




Chrisley, R. (1994) Why everything doesn't realize every computation.


   Minds and Machines 4(4):403--420.




Putnam, H. (1988) Representation and Reality.  MIT Press.




Searle, J. (1990) Is the brain a digital computer?  Proceedings and


   Addresses of the American Philosophical Association 64.




Smith, B. C. (1996) On the Origin of Objects. MIT Press