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From: harnad@mind.UUCP (Stevan Harnad)
Newsgroups: comp.ai,comp.cog-eng
Subject: Re: The symbol grounding problem
Message-ID: <918@mind.UUCP>
Date: Fri, 26-Jun-87 18:17:16 EDT
Article-I.D.: mind.918
Posted: Fri Jun 26 18:17:16 1987
Date-Received: Sat, 27-Jun-87 11:04:23 EDT
References: <764@mind.UUCP> <768@mind.UUCP> <770@mind.UUCP> <6174@diamond.BBN.COM> <6670@diamond.BBN.COM>
Organization: Cognitive Science, Princeton University
Lines: 175
Summary: 17th century bottom-upism has yet to be tested
Xref: mnetor comp.ai:578 comp.cog-eng:151



aweinste@Diamond.BBN.COM (Anders Weinstein) of BBN Laboratories, Inc.,
Cambridge, MA writes:

>	I don't see any difference between "physical" and "merely theoretical"
>	invertibility... Surely you don't mean that a transformation-inversion
>	capability must actually be present in the device for it to count as
>	"analog" in your sense.  (Else brains, for example, wouldn't count).

I think this is partly an empirical question. "Physically possible"
invertibility is enough for an analog transformation, but actual
physical invertibility may be necessary for an iconic representation
that can generate all of our discrimination capacities. Avoiding
"merely theoretical" invertibility is also part of avoiding any reliance
on mediation by our theoretical interpretations in order to get an
autonomous, intrinsically grounded system.

>	the *semantic* meaning  of a symbol is still left largely unconstrained
>	even after you take account of it's "grounding" in perceptual
>	categorization. This is because what matters for intentional content
>	is not the objective property in the world that's being detected, but
>	rather how the subject *conceives* of that external property, a far
>	more slippery notion... primitive people may be able to reliably
>	categorize certain large-scale atmospheric electrical discharges;
>	nevertheless, the semantic content of their corresponding states might
>	be "Angry gods nearby" or some such.

I agree that symbol grounding cannot be based on the "objective
property" that's being detected. Categorical representations in my
grounding model are approximate. All they do is sort and label the confusable
alternatives that have been sampled, using the provisional features
that suffice to generate reliable sorting performance according to the feedback
that defines "right" and "wrong." There is always a context of
confusable alternatives, and which features are used to sort reliably
is always a "compared to what?" matter. The exact "objective property" they
pick out is never an issue, only whether they can generate reliable
asymptotic categorization performance given that sample and those
feedback constraints. The representation is indifferent to whether
what you are calling "water," is really "twin-water" (with other
objective properties), as long as you can sort it "correctly" according
to the feedback (say, from the dictates of thirst, or a community of
categorizing instructors).

As to what people "conceive" themselves to be categorizing: My model
is proposed in a framework of methodological epiphenomenalism. I'm
interested in what's going on in people's heads only inasmuch as it is
REALLY generating their performance, not just because they think or
feel it is. So, for example, in criticizing the Roschian approach to
categorization in my reply to Dan Berleant I suggested that it was
irrelevant what features subjects BELIEVED they were using to
categorize, say, chairs; what matters is what features they (or any
organism or device in a similar input situation) really ARE using.
[This does not contradict my previous point about the irrelevance of
"objective properties." "Features" refers to properties of the
proximal projection on the device's sense receptors, whereas
"properties" would be the essential characteristics of distal objects
in the world. Feature detectors are blind to distal differences that
are not preserved in the proximal projection.]

On the other hand, "Angry gods nearby" is not just an atomic label for
"thunder" (otherwise it WOULD be equivalent to it in my model -- both
labels would pick out approximately the same thing); in fact, it is
decomposable, and hence has a different meaning in virtue of the
meanings of "angry" and "gods." There should be corresponding internal
representational differences (iconic, categorical and symbolic) that
capture that difference.

>	Another well-known obstacle to moving from an objective to an
>	intentional description is that the latter contains an essentially
>	normative component, in that we must make some distinction between
>	correct and erroneous classification. For example, we'd probably
>	like to say that a frog has a fly-detector which is sometimes wrong,
>	rather than a "moving-spot-against-a- fixed-background" detector
>	which is infallible. Again, this distinction seems to depend on fuzzy
>	considerations about the purpose or functional role of the concept
>	in question... [In his reply on this point to Dan Berleant,
>	Weinstein continues:] the philosophical problem is to say why any
>	response should count as an *error* at all. What makes it wrong?
>	I.e. who decides which "concept" -- "fly" or "moving-spot..." -- the
>	frog is trying to apply? The objective facts about the frog's
>	perceptual abilities by themselves don't seem to tell you that in
>	snapping out its tongue at a decoy, it's making a *mistake*. To
>	say this, an outside interpreter has to make some judgement about what
>	the frog's brain is trying to accomplish by its detection of moving
>	spots. And this makes the determination of semantic descriptions a
>	fuzzy matter.

I don't think there's any problem at all of what should count as an "error"
for my kind of model. The correctness or incorrectness of a label is
always determined by feedback -- either ecological, as in evolution
and daily nonverbal learning, or linguistic, where it is conventions
of usage that determine what we call what. I don't see anything fuzzy about
such a functional framework. (The frog's feedback, by the way,
probably has to do with edibility, so (i) "something that affords eating"
is probably a better "interpretation" of what it's detecting. And, to
the extent that (ii) flies and (iii) moving spots are treated indifferently by
the detector, the representation is approximate among all three.
The case is not like that of natives and thunder, since the frog's
"descriptions" are hardly decomposable. Finally, there is again no
hope of specifying distal "objective properties" ["bug"/"schmug"] here
either, as approximateness continues to prevail.)

>	Some of the things you say also suggest that you're attempting to
>	resuscitate a form of classical empricist sensory atomism, where the
>	"atomic" symbols refer to sensory categories acquired "by acquaintance"
>	and the meaning of complex symbols is built up from the atoms "by
>	description". This approach has an honorable history in philosophy;
>	unfortunately, no one has ever been able to make it work. In addition
>	to the above considerations, the main problems seem to be: first,
>	(1) that no principled distinction can be made between the simple
>	sensory concepts and the complex "theoretical" ones; and second,
>	(2) that very little that is interesting can be explicitly defined in
>	sensory terms (try, for example, "chair")...[In reply to Berleant,
>	Weinstein continues:] Of course *some* concepts can be acquired by
>	definition. However, the "classical empiricist" doctrine is committed
>	to the further idea that there is some privileged set of *purely
>	sensory* concepts and that all non-sensory concepts can be defined in
>	terms of this basis. This is what has never been shown to work. If you
>	regard "juice" as a "primitive" concept, then you do not share the
>	classical doctrine. (And if you do not, I invite you try giving
>	necessary and sufficient conditions for juicehood.)

You're absolutely right that this is a throwback to seventeenth-century
bottom-upism.  In fact, in the CP book I call the iconic and
categorical representations the "acquaintance system" and the symbolic
representations the "description system." The only difference is that
I'm only claiming to be giving a theory of categorization. Whether or
not this captures "meaning" depends (for me at any rate) largely on
whether or not such a system can successfully pass the Total Turing
Test. It's true that no one has made this approach work. But it's also
true that no one has tried. It's only in today's era of computer
modeling, robotics and bioengineering that these mechanisms will begin
to be tested to see whether or not they can deliver the goods.

To reply to your "two main problems": (1) Even an elementary sensory
category such as "red" is already abstract once you get beyond the
icon to the categorical representation. "Red" picks out the
electromagnetic wave-lengths that share the feature of being above and
below a certain threshold. That's an abstraction. And in exchange for
generating a feature-detector that reliably picks it out, you get a
label -- "red" -- which can now enter into symbolic descriptions (e.g.,
"red square"). Categorization is abstraction. As soon as you've left
the realm of invertible icons, you've begun to abstract, yet you've
never left the realm of the senses. And so it goes, bottom up, from
there onward. 

(2) As to sensory "definitions": I don't think this is the right thing
to look for, because it's too hard to find a valid "entry point" into
the bottom-up hierarchy. I doubt that "chair" or "juice" are sensory
primitives, picked out purely by sensory feature detectors. They're
probably represented by symbolic descriptions such as "things you can
sit on" and "things you can drink," and of course those are just the
coarsest of first approximations. But the scenario looks pretty
straightforward: Even though it's flexible enough to be revised to
include a chair (suitably homegenized) as a juice and a juice (for a
bug?) as a chair, it seems very clear that it is the resources of (grounded)
symbolic description that are being drawn upon here in picking out
what is and is not a chair, and on the basis of what features.

The categories are too interrelated (and approximate, and provisional) for
an exhaustive "definition," but provisional descriptions that will get
you by in your sorting and labeling -- and, more important, are
revisable and updatable, to tighten the approximation -- are certainly
available and not hard to come by. "Necessary and sufficient conditions for
juicehood," however, are a red herring. All we need is a provisional
set of features that will reliably sort the instances as environmental and
social feedback currently dictates. Remember, we're not looking for
"objective properties" or ontic essences -- just something that will
guide reliable sorting according to the contingencies sampled to date.
-- 

Stevan Harnad                                  (609) - 921 7771
{bellcore, psuvax1, seismo, rutgers, packard}  !princeton!mind!harnad
harnad%mind@princeton.csnet       harnad@mind.Princeton.EDU