How we learn language by mixing memory and rules.
The fundamental scientific problem raised by language is to explain its vast
expressive power. What is the trick behind our ability to fill each others'
heads with so many kinds of thoughts? Many language scientists, from Noam
Chomsky to his strongest critics, have sought to identify a single mental
mechanism that would explain our way with words. I will argue, on the
contrary, that language arises not from one trick but from two. They work by
different principles, are learned and used in different ways, and may even
reside in different parts of the brain. Their border disputes shape and
reshape languages over centuries, and make language not only a tool for
communication but also a medium for wordplay and an heirloom of endless
fascination.
Each "trick" was identified in the nineteenth century by Continental
linguists.
The first is the principle of the memorized word, which Ferdinand de Saussure
called the arbitrary sign. Though a memorized link between a sound and a
meaning is rather simple, it can be effective. Since human memory is vast,
we can convey a large number of concepts, simply by memorizing sounds that
are paired with them. A typical high-school graduate knows around 60,000
words, which works out to a rate of learning a new sound-meaning association
approximately every ninety waking minutes starting at the age of one. Also,
these entries require little in the way of computation.
Given the sound, you can look up the meaning (in comprehension); given a
meaning, you look up the sound (in production).
But, of course, we don't just blurt out individual words. We combine them into
phrases and sentences, and that brings up the second trick behind language,
combinatorial grammar - what Wilhelm von Humboldt called "the
infinitive use of finite media". Everyone who speaks a given language
has a recipe or algorithm for combining words in such a way that the meaning
of the combination can be deduced from the meanings of the words and the way
they are arranged. For example, one English rule says that a sentence is
composed of a subject - a noun phrase - followed by a predicate - a verb
phrase. The verb phrase in turn can consist of a verb, followed by a noun
phrase - the object - followed by a sentence, the complement.
The advantage of combinatorial grammar is that by allowing us to combine
symbols we can express new combinations of ideas. Journalists say that when
a dog bites a man, that isn't news, but when a man bites a dog, that is
news.
Grammar allows us to convey news, by reshuffling words in particular orders.
Moreover, because our knowledge of language is couched in abstract symbols -
noun, verb, subject, object - the same rules allow us to talk about a big
dog biting a man and a big bang creating a universe.
Another advantage of grammar is that the number of combinations it can
generate grows exponentially with the length of the string. If there are,
say, 10,000 nouns with which to begin a sentence, and then 4,000 words one
can use to continue it, there are 10,000 x 4,000 = 40 million two-word
beginnings to a sentence, and the number of possible sentences explodes as
you continue to add words to the tail of the growing sentence. A final
advantage is that human grammars are recursive: a sentence contains a
predicate, which can in turn contain a sentence, which can contain a
predicate, and so on. That provides an ability to generate structures of
arbitrary size, hence an unlimited number of different sentences.
I suggest that the basic design of human language combines the advantages of
these two principles. We have a lexicon of words for both common and
idiosyncratic entities like ducks and dogs and men, which depends on the
psychological mechanism called memory. And we have a set of grammatical
rules for novel combinations of entities, for dogs biting men and men biting
dogs, which depends on a mental mechanism of symbol combination.
To test this idea, we need a case in which words and rules can express the
same idea, but are psychologically, and ultimately neurologically,
distinguishable.
I believe we do have such a case: regular and irregular inflection.
Verbs in English and in many other languages come in two flavours. Regular
verbs such as "walk, walked", "jog, jogged", and "kiss,
kissed" are monotonously predictable; all form the past tense by
suffixing the stem with "-ed". The regular verbs are open-ended.
English has thousands of existing regular verbs, and new ones are being added
all the time. When "to fax" entered common parlance about fifteen
years ago, no one had to run to the dictionary to look up its past tense
form; everyone knew it was "faxed", Similarly, "flamed", "dissed",
"moshed" and "spammed" can all be deduced without having
to hear them in the past-tense form.
This productivity is visible even in children. In 1958, Jean Berko Gleason
brought some four-year-olds into the lab and said "Here is a man who
knows how to wug. He did the same thing yesterday. He . . . ." The
children filled in the blank with "wugged", a word they had never
heard before, so they could not have memorized it beforehand; they must have
generated it on the fly. And in one sense all children are subjects in an
experiment like that, because they pass through a stage in which they
produce other forms they could not have heard from their parents, forms like "comed",
"goed", "bringed", "taked", and "holded".
And that brings us to the second flavour of verbs in English, the irregular
verbs, such as "bring, brought", "hit, hit", "go,
went", "sleep, slept", "make, made", "ring,
rang" and "fly, flew". In contrast to the regulars, the
irregulars are unpredictable. The past tense of "sink" is "sank"
or "sunk", while the past tense of "cling" is "clung"
but not "clang". The past tense of "think" is neither "thank"
nor "thunk" but "thought", and the past tense of "blink"
is neither "blank" nor "blunk" nor "blought",
but is regular, "blinked".
Also unlike the regulars, the irregulars form a closed class. About 180 verbs
are irregular in standard English, and there have not been any recent new
ones.
All this leads to a simple theory. Irregular verbs are simply pairs of words.
Just as we memorize "duck", we memorize "bring" and we
memorize "brought", and then we link the two in memory. Regular
verbs are generated by a rule, akin to the rule generating sentences out of
subjects and predicates. This rule says that a verb in the past tense may be
composed of the verb stem plus the suffix "-ed". If a verb does
not supply a past-tense form from memory, the regular rule applies by
default; that is how children and adults can say things like "wugged"
and "faxed" and "spammed", which cannot have been stored
in memory beforehand.
Alas, there is a complication for this neat theory: the irregular verbs
display patterns. We find families of irregular verbs such as: "keep,
kept", "sleep, slept", "feel, felt" and "dream,
dreamt"; "wear, wore", "bear, bore", "tear,
tore" and "swear, swore"; "string, strung", "swing,
swung", "sting, stung" and "fling, flung". This is
not what we would expect if the irregular verbs were memorized individually
by rote, in which case they could just as easily all be idiosyncratic.
Moreover, these aren't just redundancies in memory; they are occasionally
generalized. Occasionally children make errors like "bring, brang",
"bite, bote" and "wipe, wope". In the history of
language, every once in a while a new irregular verb appears. "Quit"
and "knelt" are only about 200 years old (Jane Austen, for
example, used "quitted"), and "snuck", which is now
standard among Americans and Canadians under fifty, has been in the language
only for 100 years. This is especially obvious if you compare non-standard
dialects, which contain forms like "help, holp", "drag, drug"
and "bring, brung". Experimental psychologists can even catch
people in the act of generalizing an irregular pattern: when Joan Bybee and
Carol Moder asked students "What is the past tense of 'to spling'?",
many said "splang" or "splung".
How do we account for these patterns and generalizations, which are neither
clearly word-like nor clearly rule-like? Two alternatives to the
words-and-rules dichotomy have been proposed. Each tries to stretch one of
the components to cover the territory ordinarily allotted to the other.
According to the theory of generative phono-logy from Noam Chomsky and Morris
Halle, there are rules all the way down. Just as we have a rule adding "-ed"
to form the regular past tense, we have a suite of rules that generate
irregular past-tense forms by substituting vowels or consonants. For
example, one rule changes "i" to "u" in verbs like "cling,
clung".
A problem for this theory is the family resemblance among the verbs undergoing
the rule, such as "string, strung", "sting, stung", "fling,
flung", "cling, clung". How do you get the rule to apply to
them? If you simply link the rule by stipulation to each of the words, you
have no explanation for why the words are so similar. Why "string,
strung", "sting, stung", and "fling, flung", which
share the consonants before and after the "i", and not "fib,
fub", "wish, wush," and "trip, trup"? The obvious
move at this point is to distil some common pattern out of the set of words
that undergo a rule and append it to the rule as a condition. But that does
not work either. Say the rule is restricted to apply only to verbs that
begin with two consonants and end with "ng". Such a rule would
falsely include verbs like "bring" and "spring", which
fit the pattern but whose simple past-tense forms are "brought"
and "sprang" (not "brung" and "sprung"). At
the same time, the rule falsely excludes words like "stick, stuck"
and "spin, spun", which miss the condition by a whisker.
The problem is that the words showing an irregular pattern are "family-resemblance"
categories in the sense Ludwig Wittgenstein proposed. No set of properties
runs through the entire class; rather, patterns of overlapping similarities
probabilistically link various subsets.
This led to something completely different: the theory of parallel distributed
processing or artificial neural networks from David Rumelhart and James
McClelland and their followers. Rather than having words all the way down,
this theory has memory associations all the way up. Rumelhart and McClelland
devised a neural network model called a pattern-associator memory, which
links not an item to an item, but the features of an item - the sounds
composing it - to the features of an item. A word is presented to the
network by turning on units corresponding to the word's sounds. The model is
trained with examples of a verb and its past-tense form: "sing, sang",
"walk, walked", and so on. It records correlations between
features of the stem and features of the past-tense form, and that allows it
to generalize a pattern to a new verb if it is similar to verbs it has been
trained on: once trained on "ring" and "sing" and "spring"
and "cling", it automatically generalizes to "spling",
because some of the pieces of "spling" occupy the same
representational real estate as the pieces of "ring" and "sing".
In the same way, it generalizes from "walked" and "talked"
to "balked" and "stalked". It generalizes reasonably
well, despite not having any distinction between words and rules; a single
mechanism handles regular and irregular forms.
