Reply to S. Harnad's questions, short version

[Steve Pinker]

Alluding to our paper "On Language and Connectionism: Analysis of a
PDP model of language acquisition", Stevan Harnad has posted a list of
questions and observations as a 'challenge' to us.  His remarks owe
more to the general ambience of the connectionism / symbol-processing
debate than to the actual text of our paper, in which the questions
are already answered. We urge those interested in these issues to read
the paper or the nutshell version published in Trends in
Neurosciences, either of which may be obtained from Prince (address
below). In this note we briefly answer Harnad's three questions. In
another longer message to follow, we direct an open letter to Harnad
which justifies the answers and goes over the issues he raises in more
detail.

Question # 1: Do we believe that English past tense formation is not
learnable?  Of course we don't! So imperturbable is our faith in the
learnability of this system that we ourselves propose a way in which
it might be done (OLC, 130-136).

Question #2: If it is learnable, is it specifically unlearnable by
nets? No, there may be some nets that can learn it; certainly any net
that is intentionally wired up to behave exactly like a rule-learning
algorithm can learn it. Our concern is not with (the mathematical
question of) what nets can or cannot do in principle, but with which
theories are true, and our conclusions were about pattern associators
using distributed phonological representations.  We showed that it is
unlikely that human children learn the regular rule the way such a
pattern associator learns the regular rule, because it is simply the
wrong tool for the job.  Therefore it's not surprising that the
developmental data confirm that children do not behave in the way that
such a pattern associator behaves.

Question # 3: If past tense formation is learnable by nets, but only
if the invariance that the net learns and that causally constrains its
successful performance is describable as a "rule", what's wrong with
that?  Absolutely nothing! --just like there's nothing wrong with
saying that past tense formation is learnable by a bunch of
precisely-arranged molecules (viz., the brain) but only if the
invariance that the molecules learn, etc. etc. etc.  The question is,
what explains the facts of human cognition?  Pattern associator
networks have some interesting properties that can shed light on
certain kinds of phenomena, such as *irregular* past tense forms. But
it is simply a fact about the *regular* past tense alternation in
English that it is not that kind of phenomenon.  You can focus on the
interesting empirical predictions of pattern associators, and use them
to explain certain things (but not others), or you can generalize them
to a class of universal devices that can explain nothing without an
appeal to the rules that they happen to implement. But you can't have
it both ways.

Alan Prince
Program in Cognitive Science
Department of Psychology
Brown 125
Brandeis University
Waltham, MA 02254-9110
prince at brandeis.bitnet

Steven Pinker
Department of Brain and Cognitive Sciences
E10-018
MIT
Cambridge, MA 02139
steve at cogito.mit.edu

References:

Pinker, S. & Prince, A. (1988) On language and connectionism: Analysis
of a parallel distributed processing model of language acquisition.
Cognition, 28, 73-193. Reprinted in S. Pinker & J.  Mehler (Eds.),
Connections and symbols. Cambridge, MA: Bradford Books/MIT Press.

Prince, A. & Pinker, S. (1988) Rules and connections in human
language. Trends in Neurosciences, 11, 195-202.

Rumelhart, D. E. & McClelland, J. L. (1986) On learning the past
tenses of English verbs. In J. L. McClelland, D. E. Rumelhart, & The
PDP Research Group, Parallel distributed processing: Explorations in
the microstructure of cognition. Volume 2: Psychological and
biological models. Cambridge, MA: Bradford Books/MIT Press.